Additional test stabilization (#935): Drain Queue Before Verifying Results in AsyncBlockTests

[jhugard]

Summary

This PR fixes intermittent test failures in AsyncBlockTests by ensuring the task queue is fully drained before verifying async opcodes. The race condition occurred because the Cleanup opcode is recorded asynchronously, and tests were taking opcode snapshots before that work completed. Fixes issue introduced in #935 as well as existing issue in other tests.

Changes

Apply consistent queue drain pattern to 8 test methods that verify async completion state. Comprehensive audit found both opcode verification races (4 tests) and queue empty verification races (4 additional tests).

Root Cause

The async framework's Cleanup operation is initiated by the provider but completed asynchronously through the task queue. Tests that checked queue state or opcode snapshots immediately after XAsyncGetStatus() could race with the pending Cleanup opcode write, resulting in:

• Opcode snapshot race: Missing Cleanup opcode in snapshot (8 vs 9)
• Queue empty race: Pending cleanup work leaves queue non-empty
• Intermittent failures in CI (heisenbug-like behavior)
• Confusing debug output showing inconsistent test results

Testing

✅ Comprehensive test audit completed:

• Identified 8 tests vulnerable to timing races
• Applied consistent queue drain pattern to all 8
• Verified 2 tests use MANUAL mode (no drain needed)
• Confirmed 13 other tests have no vulnerabilities

✅ All 23 AsyncBlockTests pass after fixes
✅ No regressions in other test suites
✅ Validated with extended soak testing (743 full test suite passes under page heap)
✅ Pattern aligns with existing drain waits in VerifyCleanupWaitsForWork and VerifyCleanupWaitsForWorkDistributed

Related Work

This is part of the async test stabilization effort:

• Flaky async/task-queue unit tests due to timing assumptions and use-after-free bugs #932: Data race in VerifyAsyncBlockReuse → lock-free opcode recording (commit b872e39)
• Data Race in AsyncBlockTests::VerifyAsyncBlockReuse #938: Concurrent vector mutation → atomic array with proper memory ordering (commit b872e39)
• Stabilize unit tests (#932, #938) #935: Timing race in verification → queue drain before snapshot (this PR)

Notes

• The queue drain pattern (busy-wait with 10ms sleep, 2000ms timeout) is already used in VerifyCleanupWaitsForWork tests
• No API changes or behavioral modifications to the async framework itself
• Purely a test reliability improvement in the verification layer

Checklist

• Code follows existing patterns in the test suite
• All tests pass locally
• No new warnings or errors introduced
• Consistent with upstream test stabilization work
• Soak testing validates fix is robust