C#: Migrate our own solution file to .slnx.

[michaelnebel]

In this PR we migrate our own solution file to .slnx. After the migration it becomes evident that we forgot to exclude some of the test projects from the release configuration. As we use bazel for building the extractor, I don't think it means anything in practice - adding the test project exclusions for consistency.

[Copilot]

Pull request overview

This PR migrates the C# solution file from the traditional .sln format to the newer .slnx format (XML-based). The migration ensures all 15 projects are properly included and corrects a configuration oversight where two test projects were previously included in Release builds.

Key changes:

• Replaced CSharp.sln with CSharp.slnx using XML format
• Excluded all four test projects from Release configuration (previously only 2 were excluded)
• Updated VS Code settings to reference the new .slnx file

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
csharp/CSharp.slnx	New XML-based solution file with all 15 projects and proper Release build exclusions for test projects
csharp/CSharp.sln	Removed legacy solution file
csharp/.vscode/settings.json	Updated default solution reference from CSharp.sln to CSharp.slnx

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The migration appears to be complete and correct. All projects from the original solution are present in the new .slnx file, and the test project exclusions for Release builds are now consistent across all test projects. The VS Code settings have been appropriately updated to reference the new solution file.

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