fix: make thread assertion tests deterministic

[thomhurst]

Summary

• Fix flaky Test_Thread_IsNotThreadPoolThread_NewThread test that was failing with ThreadStateException: Thread is dead; state cannot be accessed
• Replace Thread.Sleep() with ManualResetEventSlim synchronization in Test_Thread_IsAlive_NewThread, Test_Thread_IsBackground_Started, and Test_Thread_IsNotThreadPoolThread_NewThread
• Remove duplicate using TUnit.Assertions.Extensions; import

Test plan

• Verify the thread assertion tests pass on CI (macos-latest and windows-latest)
• Verify no regressions in other tests

🤖 Generated with Claude Code

[thomhurst]

Summary

Fixes flaky thread assertion tests by replacing timing-based synchronization (Thread.Sleep) with deterministic event-based synchronization (ManualResetEventSlim).

Critical Issues

None found ✅

The changes correctly address the race condition that was causing ThreadStateException: Thread is dead; state cannot be accessed. The new pattern ensures:

• Threads are guaranteed to be running before assertions via startSignal.Wait()
• Threads cleanly exit via completeSignal.Set() in finally blocks
• No timing dependencies that could cause flakiness

Suggestions

None - the implementation is clean and follows proper synchronization patterns.

Verdict

✅ APPROVE - No critical issues

The PR fixes a legitimate test flakiness issue with proper synchronization primitives. The duplicate import removal is a nice cleanup bonus.