[XSG] Use absolute file paths when generating #line

[simonrozsival]

Description of Change

We must generate absolute file paths in #line directives. Using relative paths in this context is not supported (dotnet/roslyn#71202 (comment)).

I introduced a sourcegen test but I'm not sure if this is the best way to test it. The Diagnostic instances reported by the source gen driver don't have the full paths.

This is the output of build compilation before and after the change:

# before
/Users/simonrozsival/Projects/playground/TestApp/obj/Debug/net10.0-android/Microsoft.Maui.Controls.SourceGen/Microsoft.Maui.Controls.SourceGen.CodeBehindGenerator/App.xaml(9,53): error CS1009: Unrecognized escape sequence

# after
/Users/simonrozsival/Projects/playground/TestApp/App.xaml(9,53): error CS1009: Unrecognized escape sequence

Issues Fixed

Fixes #31454

[Copilot]

Pull Request Overview

This PR fixes the generation of #line directives in the MAUI source generator to use absolute file paths instead of relative paths. Relative paths in #line directives have undefined behavior according to the Roslyn compiler, causing issues with error reporting that shows incorrect file paths in the build output.

Key changes:

• Modified the PrePost.NewLineInfo method to accept a ProjectItem instead of a string filename and use absolute paths
• Added an AbsolutePath property to ProjectItem to expose the absolute file path
• Updated all callers to pass the full ProjectItem instead of just the relative path
• Added unit tests to verify the correct absolute path generation in #line directives

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Controls/src/SourceGen/PrePost.cs	Modified to use absolute paths in #line directives by accepting ProjectItem parameter
src/Controls/src/SourceGen/ProjectItem.cs	Added AbsolutePath property and refactored to store AdditionalText reference
src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs	Updated calls to pass ProjectItem instead of relative path string
src/Controls/tests/SourceGen.UnitTests/InitializeComponent/LineInfoTests.cs	New test class to verify absolute paths in generated #line directives
src/Controls/tests/SourceGen.UnitTests/InitializeComponent/SourceGenXamlInitializeComponentTests.cs	Updated test helper to use absolute paths for consistent test behavior