Refactor C# test workflows to match pytest runner pattern (#37)

- Convert test-cs-nunit.yml and test-cs-xunit.yml to standalone runners
- Add cgroup-aware auto worker detection for parallel testing
- Add parallel_workers input with runner-based defaults (6 for multithreaded, 1 for singlethreaded)
- Standardize outputs (total, passed, percentage, collection_errors, etc.)
- Generate test_data.json artifact compatible with regression-test.yml
- NUnit uses NUnit.NumberOfTestWorkers for parallelization
- xUnit uses xUnit.MaxParallelThreads for parallelization

Co-authored-by: Claude <noreply@anthropic.com>

Author: JamesonRGrieve