Fix flaky stuck-grain deactivation test on slow runners

[ReubenBond]

Summary

Fixes flaky StuckGrainTest_StuckDetectionOnDeactivation failures on slow macOS runners.

What changed

• Replaced strict per-call timeout assertions for the first 3 calls with non-blocking enqueue behavior checks.
• Added TestingUtils.WaitUntilAsync assertion to deterministically verify those 3 calls are forwarded and processed.
• Kept the final assertion that total processed calls is 4 after an additional call.

Validation

• dotnet test .\test\TesterInternal\TesterInternal.csproj --filter "FullyQualifiedName~UnitTests.StuckGrainTests.StuckGrainTests" --no-restore --tl:off -v minimal
• Repeated StuckGrainTest_StuckDetectionOnDeactivation 5x on net8.0 with all passing.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a flaky test (StuckGrainTest_StuckDetectionOnDeactivation) that was failing on slow macOS runners by replacing strict per-call timeout assertions with a more lenient approach.

Changes:

• Replaced Assert.ThrowsAsync<TimeoutException> for each of 3 calls with non-blocking Task.WhenAny pattern
• Added TestingUtils.WaitUntilAsync polling assertion to verify the 3 calls are eventually processed
• Maintained the final assertion that 4 total calls are processed after an additional awaited call