test: migrate support/v2 tests to xUnit v3 and MTP

[Gavin Barron (gavinbarron)]

Summary

• Migrate Reader tests to xUnit v3/Microsoft.Testing.Platform using support/v2 Hidi's existing dependency versions; preserve .NET 8 Reader tests, .NET 10 Hidi tests, strong-name signing and test-source classification.
• Run both test projects through a single solution-wide test command in GitHub Actions and both Azure pipelines, with TRX publication and platform-specific reporting.
• Preserve measured OpenCover/Cobertura coverage, update SonarCloud report paths, and align local build scripts, VS Code tasks and contributor documentation.
• Leave component release triggers, signing and deployment gates unchanged.

Validation

• Debug and Release solution builds succeed with zero errors.
• Solution-wide MTP runs pass all 1,377 tests: 1,286 Reader + 91 Hidi, zero failures or skips, matching the pre-migration baseline.
• Verified separate TRX reports, both GitHub test summaries, and measured OpenCover/Cobertura coverage for the intended product assemblies.
• Verified empty discovery fails with exit code 9; checked pipeline YAML, task wiring, signing/test classification and unchanged release gates.
• Existing obsolete-source and advisory xUnit cancellation warnings remain. Hosted CI/Sonar uploads and official signing were not executed locally.

Refs #885

This PR targets support/v2 only. Main-branch work must wait until this PR is merged, then cherry-pick the actual squash merge commit, not the original feature commit.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[Gavin Barron (gavinbarron)]

SonarQube is failing us for changing test code. This is a divide by zero problem, ignore the coverage gate complaint