test(csharp): add comprehensive tests for straggler mitigation

[msrathore-db]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/straggler-config-foundation [Files changed]
  • stack/straggler-metrics [Files changed]
    • stack/straggler-detector [Files changed]
      • stack/straggler-downloader-impl [Files changed]
        • stack/straggler-tests [Files changed]
          • stack/straggler-docs [Files changed]

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Summary

Adds comprehensive test coverage for the straggler mitigation feature, validating both the implementation and edge cases.

E2E Tests (19 tests):

• ✅ Slow download identification and cancellation
• ✅ Fast downloads not marked as stragglers
• ✅ Minimum completion quantile requirement
• ✅ Sequential fallback activation after threshold
• ✅ Sequential mode enforcement (one download at a time)
• ✅ No stragglers detected in sequential mode
• ✅ Sequential fallback applies only to current batch
• ✅ Monitoring thread respects cancellation
• ✅ Parallel mode respects max parallel downloads
• ✅ Cancelled straggler retry logic
• ✅ Mixed speed download scenarios
• ✅ Clean shutdown during monitoring

Advanced Unit Tests:

• ✅ Duplicate detection prevention across multiple cycles
• ✅ CancellationTokenSource atomic replacement
• ✅ Concurrent CTS cleanup
• ✅ Cleanup behavior under exceptions
• ✅ Shutdown cancellation respect
• ✅ Counter overflow protection (long type)
• ✅ Concurrent modification safety

Test Coverage:

• Basic functionality validation
• Edge case handling
• Thread safety verification
• Resource cleanup verification
• Cancellation token management