C#: Support .slnx. solution files.

[michaelnebel]

In this PR we implement support for the .slnx file format - both for build-mode: none and autobuilder.

Even though it is discouraged to use SolutionFile.Parse (it is considered an internal implementation detail), it appears that parsing support for .slnx was introduced here. If we wan't to do it the supported way, we would basically need to copy the parsing implementation. IMO we should continue to use the exposed parsing functionality and only mirror the implementation in case it becomes deprecated.
We need to explicitly reference Microsoft.VisualStudio.SolutionPersistence for the new parsing to be applied.

This feature is now more crucial as .NET 10 uses the .slnx format as default, which is explained here

[Copilot]

Pull request overview

This PR adds support for .slnx (XML-based solution) files in the C# CodeQL analysis for both build-mode: none and autobuilder scenarios. The implementation leverages the Microsoft.VisualStudio.SolutionPersistence NuGet package to parse .slnx files using the existing SolutionFile.Parse() API.

Key changes:

• Added .slnx to supported file extensions in documentation
• Created integration tests for both standalone and autobuilder scenarios with .slnx files
• Updated dependency files to include Microsoft.VisualStudio.SolutionPersistence package

Reviewed changes

Copilot reviewed 31 out of 33 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/codeql/reusables/supported-versions-compilers.rst	Added .slnx to the list of supported file extensions for C#
csharp/ql/lib/change-notes/2025-12-11-slnx-support.md	Added change note documenting the new feature
csharp/ql/integration-tests/posix/standalone_dependencies_executing_runtime/Assemblies.expected	Updated expected assemblies to include the new SolutionPersistence DLL
csharp/ql/integration-tests/all-platforms/standalone_slnx/*	New integration test suite for standalone extraction with .slnx files
csharp/ql/integration-tests/all-platforms/autobuild_slnx/*	New integration test suite for autobuilder with .slnx files
csharp/paket.main.bzl	Updated package versions and added Microsoft.VisualStudio.SolutionPersistence dependency
csharp/paket.lock	Updated lock file with new package versions
csharp/paket.dependencies	Added Microsoft.VisualStudio.SolutionPersistence package reference
csharp/extractor/Semmle.Util/Language.cs	Enhanced Language class to support solution-specific extensions; refactored constructors with parameter reordering
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/paket.references	Added SolutionPersistence package reference
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/SolutionFile.cs	Updated comments to mention .slnx support
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/FileProvider.cs	Extended solution file search to include .slnx extension
csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/BUILD.bazel	Added SolutionPersistence dependency
csharp/autobuilder/Semmle.Autobuild.Shared/paket.references	Added SolutionPersistence package reference
csharp/autobuilder/Semmle.Autobuild.Shared/Solution.cs	Updated comments to mention .slnx support
csharp/autobuilder/Semmle.Autobuild.Shared/BUILD.bazel	Added SolutionPersistence dependency
csharp/autobuilder/Semmle.Autobuild.Shared/Autobuilder.cs	Added logic to search for language-specific solution files (.slnx) in build priority

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[Copilot]

The parameter order has been changed in the constructor, which could be a breaking change if this constructor is called from other places in the codebase. However, this appears to be intentional to make the API more readable with the name parameter first. Note that both constructors have their parameters swapped from (extension, name) to (name, extension).

[michaelnebel]

The constructors are private.

[michaelnebel]

DCA looks good.