Add Codecov coverage for C# SDK

[hubcio]

Rust and Java already upload coverage - C# is missing. Follow the same pattern. Microsoft.Testing.Extensions.CodeCoverage is already a dependency.

• Add --collect:"XPlat Code Coverage" to dotnet test calls in .github/actions/csharp-dotnet/pre-merge/action.yml and merge into a single Cobertura XML
• Add codecov/codecov-action@v5 upload step in .github/workflows/_test.yml for sdk-csharp with flags: csharp
• Add post-merge baseline job in .github/workflows/post-merge.yml (same pattern as java-coverage — start server, run tests with coverage, upload)
• Add csharp flag to codecov.yml (paths: foreign/csharp/) and remove foreign/csharp/** from ignore