.NET: Enforce .NET PR Test Coverage Gate

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.