Fix XSLT format-date and format-time to output only date or time with empty format string (#121028)

Fixing XSLT extension methods format-date & format-time to respect the
isDate parameter when using empty format string.

**Plan:**
- [x] Analyze the issue and understand the problem
- [x] Locate the problematic code in XsltFunctions.cs
- [x] Create focused tests for format-date and format-time functions
- [x] Fix the MSFormatDateTime method to use the isDate parameter
correctly
- [x] Build and test the changes
- [x] Verify the fix resolves the issue
- [x] Run CodeQL security check
- [x] Fix culture-dependent test failures
- [x] Remove tests that don't work on .NET Framework
- [x] Use ThreadCultureChange helper and improve code style
- [x] Optimize CultureInfo handling to avoid creating it twice
- [x] Improve test assertions to be more precise

**Issue Summary:**
The MSFormatDateTime function in XsltFunctions.cs ignored the `isDate`
parameter when the format string was empty. This caused both
format-date() and format-time() XSLT functions to output both date and
time instead of just the date or just the time respectively when using
the default (empty) format string.

**Changes Made:**
1. Added 2 focused tests in XslCompilerTests.cs to verify the fix for
empty format strings:
- FormatDateWithEmptyFormatString: Verifies date-only output with empty
format
- FormatTimeWithEmptyFormatString: Verifies time-only output with empty
format
   - Tests now use ThreadCultureChange helper for cleaner code
   - String constants use PascalCase naming
   - Tests compare against expected formatted values for precision

2. Fixed MSFormatDateTime in XsltFunctions.cs:
- When format string is empty, now uses "d" (short date pattern) for
dates
- When format string is empty, now uses "T" (long time pattern) for
times
- When format string is not empty, uses the provided format (unchanged
behavior)
   - Optimized to reuse CultureInfo object instead of creating it twice

3. Removed tests with explicit format strings that were failing on .NET
Framework

**Test Results:**
- All 3 XslCompilerTests tests pass ✓
- All 1618 existing XslCompiledTransformApi tests pass ✓
- Tests focus specifically on the empty format string issue ✓
- No security vulnerabilities detected ✓

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<details>

<summary>Original prompt</summary>


----

*This section details on the original issue you should resolve*

<issue_title>XSLT extension methods format-date & format-time both
output date & time when using the empty string (default)
format</issue_title>
<issue_description>### Description

I'm porting a .NET FW application to .NET (6 and above).
The application performs an XML to HTML conversion using an XSLT
transform via System.Xml.Xsl.XslCompiledTransform.

Under .NET FW, this XSL line:

`<xsl:value-of select="ms:format-date('2001-02-03T01:02:03', '')"/>`

Produces:

`03/02/2001`

UK date format since that's my locale.

However, under .NET 6 (I've also tried 8), it produces:

`03/02/2001 01:02:03`

Similarly, the format-time function also outputs both the date & time.

### Reproduction Steps

It's easy to see there's a problem from the identified source code.

### Expected behavior

The default (empty) format string should cause format-date() to output
just the date, and correspondingly, format-time() to output just the
time - as was the case under .NET FW and is how the functions are
documented.

### Actual behavior

Both functions output both the date and time.

### Regression?

Yes, the functions works as documented for .NET FW.

### Known Workarounds

Don't use the default (empty) format string - pass either "d" or "T" to
the functions.

### Configuration

.NET 6 and above (I've not tried anything < 6)


### Other information

Looking at the .NET source code, the issue seems to be in the
[MSFormatDateTime
function](https://github.com/dotnet/runtime/blob/b21d709640e52cf43dc78a561e93174c7279e3c5/src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/XsltFunctions.cs#L365).
Note that the function doesn't use the "isDate" parameter.

Compare that code with the [.NET FW
version](https://referencesource.microsoft.com/#System.Data.SqlXml/System/Xml/Xsl/Runtime/XsltFunctions.cs,340)
which clearly uses the isDate parameter to do just the Date or Time
appropriately.</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@</author><body>
Tagging subscribers to this area: @dotnet/area-system-xml
See info in
[area-owners.md](https://github.com/dotnet/runtime/blob/main/docs/area-owners.md)
if you want to be subscribed.
<details>
<summary>Issue Details</summary>
<hr />

### Description

I'm porting a .NET FW application to .NET (6 and above).
The application performs an XML to HTML conversion using an XSLT
transform via System.Xml.Xsl.XslCompiledTransform.

Under .NET FW, this XSL line:

`<xsl:value-of select="ms:format-date('2001-02-03T01:02:03', '')"/>`

Produces:

`03/02/2001`

UK date format since that's my locale.

However, under .NET 6 (I've also tried 8), it produces:

`03/02/2001 01:02:03`

Similarly, the format-time function also outputs both the date & time.

### Reproduction Steps

It's easy to see there's a problem from the identified source code.

### Expected behavior

The default (empty) format string should cause format-date() to output
just the date, and correspondingly, format-time() to output just the
time - as was the case under .NET FW and is how the functions are
documented.

### Actual behavior

Both functions output both the date and time.

### Regression?

Yes, the functions works as documented for .NET FW.

### Known Workarounds

Don't use the default (empty) format string - pass either "d" or "T" to
the functions.

### Configuration

.NET 6 and above (I've not tried anything < 6)


### Other information

Looking at the .NET source code, the issue seems to be in the
[MSFormatDateTime
function](https://github.com/dotnet/runtime/blob/b21d709640e52cf43dc78a561e93174c7279e3c5/src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/XsltFunctions.cs#L365).
Note that the function doesn't use the "isDate" parameter.

Compare that code with the [.NET FW
version](https://referencesource.microsoft.com/#System.Data.SqlXml/System/Xml/Xsl/Runtime/XsltFunctions.cs,340)
which clearly uses the isDate parameter to do just the Date or Time
appropriately.

<table>
  <tr>
    <th align="left">Author:</th>
    <td>Dave-Lowndes</td>
  </tr>
  <tr>
    <th align="left">Assignees:</th>
    <td>-</td>
  </tr>
  <tr>
    <th align="left">Labels:</th>
    <td>

`area-System.Xml`, `untriaged`

</td>
  </tr>
  <tr>
    <th align="left">Milestone:</th>
    <td>-</td>
  </tr>
</table>
</details></body></comment_new>
<comment_new><author>@krwq</author><body>
It does seem like a bug @Dave-Lowndes . It seems this got regressed
here:
d784b77

We should figure out why that change was made and make sure we don't
regress issue being fixed there while fixing this.

@Dave-Lowndes are you perhaps interested in making a pull
request?</body></comment_new>
<comment_new><author>@krwq</author><body>
I agree that would be ideal but they're no longer on our team (and even
if I doubt anyone...

</details>

Fixes #93189

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Author: 3 people

src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/XsltFunctions.cs

@@ -366,7 +366,7 @@ public static string MSFormatDateTime(string dateTime, string format, string lan
         {
             try
             {
-                string locale = GetCultureInfo(lang).Name;
+                CultureInfo ci = GetCultureInfo(lang);
 
                 XsdDateTime xdt;
                 if (!XsdDateTime.TryParse(dateTime, XsdDateTimeFlags.AllXsd | XsdDateTimeFlags.XdrDateTime | XsdDateTimeFlags.XdrTimeNoTz, out xdt))
@@ -375,8 +375,13 @@ public static string MSFormatDateTime(string dateTime, string format, string lan
                 }
                 DateTime dt = xdt.ToZulu();
 
-                // If format is the empty string or not specified, use the default format for the given locale
-                return dt.ToString(format.Length != 0 ? format : null, new CultureInfo(locale));
+                // If format is the empty string or not specified, use the default date or time format for the given locale
+                if (format.Length == 0)
+                {
+                    format = isDate ? "d" : "T";
+                }
+
+                return dt.ToString(format, ci);
             }
             catch (ArgumentException)
             { // Operations with DateTime can throw this exception eventualy

src/libraries/System.Private.Xml/tests/Xslt/XslCompiledTransformApi/XslCompilerTests.cs

@@ -1,7 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Globalization;
 using System.IO;
+using System.Tests;
 using System.Xml.Xsl;
 using Xunit;
 
@@ -44,5 +46,71 @@ public void ValueOfInDebugMode()
                 Assert.Equal("This is my class info", outWriter.ToString());
             }
         }
+
+        [Fact]
+        public void FormatDateWithEmptyFormatString()
+        {
+            const string Xml = @"<?xml version=""1.0"" encoding=""UTF-8""?><root/>";
+            const string Xsl = @"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform""
+                    xmlns:ms=""urn:schemas-microsoft-com:xslt"">
+    <xsl:output method=""text"" />
+    <xsl:template match=""/"">
+        <xsl:value-of select=""ms:format-date('2001-02-03T01:02:03', '')"" />
+    </xsl:template>
+</xsl:stylesheet>";
+
+            using (new ThreadCultureChange("en-GB"))
+            using (var outWriter = new StringWriter())
+            {
+                using (var xslStringReader = new StringReader(Xsl))
+                using (var xmlStringReader = new StringReader(Xml))
+                using (var xslReader = XmlReader.Create(xslStringReader))
+                using (var xmlReader = XmlReader.Create(xmlStringReader))
+                {
+                    var transform = new XslCompiledTransform();
+                    transform.Load(xslReader);
+                    transform.Transform(xmlReader, null, outWriter);
+                }
+
+                string result = outWriter.ToString();
+                DateTime expectedDate = new DateTime(2001, 2, 3, 1, 2, 3, DateTimeKind.Utc);
+                string expectedResult = expectedDate.ToString("d", new CultureInfo("en-GB"));
+                Assert.Equal(expectedResult, result);
+            }
+        }
+
+        [Fact]
+        public void FormatTimeWithEmptyFormatString()
+        {
+            const string Xml = @"<?xml version=""1.0"" encoding=""UTF-8""?><root/>";
+            const string Xsl = @"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform""
+                    xmlns:ms=""urn:schemas-microsoft-com:xslt"">
+    <xsl:output method=""text"" />
+    <xsl:template match=""/"">
+        <xsl:value-of select=""ms:format-time('2001-02-03T01:02:03', '')"" />
+    </xsl:template>
+</xsl:stylesheet>";
+
+            using (new ThreadCultureChange("en-GB"))
+            using (var outWriter = new StringWriter())
+            {
+                using (var xslStringReader = new StringReader(Xsl))
+                using (var xmlStringReader = new StringReader(Xml))
+                using (var xslReader = XmlReader.Create(xslStringReader))
+                using (var xmlReader = XmlReader.Create(xmlStringReader))
+                {
+                    var transform = new XslCompiledTransform();
+                    transform.Load(xslReader);
+                    transform.Transform(xmlReader, null, outWriter);
+                }
+
+                string result = outWriter.ToString();
+                DateTime expectedTime = new DateTime(2001, 2, 3, 1, 2, 3, DateTimeKind.Utc);
+                string expectedResult = expectedTime.ToString("T", new CultureInfo("en-GB"));
+                Assert.Equal(expectedResult, result);
+            }
+        }
+
+
     }
 }