[WinHTTP] Remove legacy (Win 8.1-) WinInet proxy handling

[ManickaP]

Usage of WinInetProxyHelper was relevant only for legacy Windows 8.1-. Since 8.1, WinHTTP handles per-user proxies itself, see https://learn.microsoft.com/en-us/windows/win32/api/winhttp/nf-winhttp-winhttpopen.

Removed the helper from WinHttpHandler and moved the few tests to S.N.Http.UnitTests next to the other HttpWindowsProxy tests.

Note that HttpWindowsProxy is the last user of WinInetProxyHelper so I moved the source file from Common. I can move it back there if we think this has a potential for being shared.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

This PR removes the legacy WinInet proxy handling that was only used for Windows 8.1 and earlier. The key changes include:

• Removing the WinInetProxyHelper usage and related legacy fallback code in WinHttpHandler.
• Migrating and adding tests for HttpWindowsProxy in System.Net.Http.UnitTests.
• Eliminating obsolete tests and TestControl properties from the WinHttpHandler unit tests.

Reviewed Changes

Copilot reviewed 7 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Net.Http/tests/UnitTests/HttpWindowsProxyTest.cs	Added new tests for verifying manual proxy settings with auto-detection failures and manual-only scenarios.
src/libraries/System.Net.Http.WinHttpHandler/tests/UnitTests/WinHttpHandlerTest.cs	Removed tests that relied on the legacy WinInet proxy handling and the WinHttpAutomaticProxySupport property.
src/libraries/System.Net.Http.WinHttpHandler/tests/UnitTests/TestControl.cs	Removed obsolete WinHttpAutomaticProxySupport property.
src/libraries/System.Net.Http.WinHttpHandler/tests/UnitTests/FakeInterop.cs	Removed legacy branch handling based on WinHttpAutomaticProxySupport.
src/libraries/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinHttpHandler.cs	Stripped legacy fallback logic for WinInet proxy detection and updated proxy option handling.

Files not reviewed (4)

• src/libraries/System.Net.Http.WinHttpHandler/src/System.Net.Http.WinHttpHandler.csproj: Language not supported
• src/libraries/System.Net.Http.WinHttpHandler/tests/UnitTests/System.Net.Http.WinHttpHandler.Unit.Tests.csproj: Language not supported
• src/libraries/System.Net.Http/src/System.Net.Http.csproj: Language not supported
• src/libraries/System.Net.Http/tests/UnitTests/System.Net.Http.Unit.Tests.csproj: Language not supported

Comments suppressed due to low confidence (1)

src/libraries/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinHttpHandler.cs:815

• Consider adding a comment here to clearly explain that the legacy fallback to WinInetProxyHelper has been removed, which will help future maintainers understand this intentional change.

ThrowOnInvalidHandle(sessionHandle, nameof(Interop.WinHttp.WinHttpOpen));