.NET: Enforce .NET PR Test Coverage Gate

[TaoChenOSU]

Implement a GitHub Action that gates pull requests for the .NET codebase to prevent coverage regression. The action should:

• Run on every PR
• Collect and compare the code coverage from the build/test workflow
• Fail the PR check if the coverage drops below the current baseline
• Provide a comment or summary on the PR about any coverage changes

This is necessary to ensure test coverage does not regress over time.