Add test to reproduce panic in onTypeFormatting with empty file

Co-authored-by: jakebailey <[email protected]>

Author: Copilot

internal/ls/format_test.go

@@ -0,0 +1,70 @@
+package ls
+
+import (
+	"context"
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/ast"
+	"github.com/microsoft/typescript-go/internal/core"
+	"github.com/microsoft/typescript-go/internal/format"
+	"github.com/microsoft/typescript-go/internal/parser"
+)
+
+// Test for issue: Panic Handling textDocument/onTypeFormatting
+// This reproduces the panic when pressing enter in an empty file
+func TestGetFormattingEditsAfterKeystroke_EmptyFile(t *testing.T) {
+	// Create an empty file
+	text := ""
+	sourceFile := parser.ParseSourceFile(ast.SourceFileParseOptions{
+		FileName: "/index.ts",
+		Path:     "/index.ts",
+	}, text, core.ScriptKindTS)
+
+	// Create language service with nil program (we're only testing the formatting function)
+	langService := &LanguageService{}
+
+	// Test formatting after keystroke with newline character at position 0
+	ctx := context.Background()
+	options := format.GetDefaultFormatCodeSettings("\n")
+
+	// This should not panic
+	edits := langService.getFormattingEditsAfterKeystroke(
+		ctx,
+		sourceFile,
+		options,
+		0, // position
+		"\n",
+	)
+
+	// Should return nil or empty edits, not panic
+	_ = edits
+}
+
+// Test with a simple statement
+func TestGetFormattingEditsAfterKeystroke_SimpleStatement(t *testing.T) {
+	// Create a file with a simple statement
+	text := "const x = 1"
+	sourceFile := parser.ParseSourceFile(ast.SourceFileParseOptions{
+		FileName: "/index.ts",
+		Path:     "/index.ts",
+	}, text, core.ScriptKindTS)
+
+	// Create language service with nil program
+	langService := &LanguageService{}
+
+	// Test formatting after keystroke with newline character at end of statement
+	ctx := context.Background()
+	options := format.GetDefaultFormatCodeSettings("\n")
+
+	// This should not panic
+	edits := langService.getFormattingEditsAfterKeystroke(
+		ctx,
+		sourceFile,
+		options,
+		len(text), // position at end of file
+		"\n",
+	)
+
+	// Should return nil or empty edits, not panic
+	_ = edits
+}