fix: remove remaining flaky wall-clock test assertions

[rogercloud]

Nine remaining tests could fail when a CI worker was descheduled: four event-loop tests used short sleeps or ticker counts, and five timeout tests imposed 3–5 second wall-clock ceilings.

Replace the event-loop timing assertions with worker-entry and release events (or the existing pool checkout gate). Keep checking timeout outcomes and configured deadlines, with 30-second test guards for hangs. Preserve the thread-CPU performance regression tests.

Validation: 144 relevant tests passed across two runs; Ruff lint, format checks, and git diff --check passed. The API run used an isolated import path because the shared editable virtualenv otherwise loaded both checkouts.

Follow-up to #2156, which merged while this audit was in progress.

[gemini-code-assist]

Code Review

This pull request refactors several asynchronous tests to eliminate flaky time-based assertions and background ticker tasks. Instead of relying on arbitrary sleep durations and measuring elapsed time, the tests now use deterministic synchronization primitives such as threading.Event and asyncio.Event along with explicit pytest.mark.timeout decorators. This ensures robust and reliable test execution across different environments. There are no review comments to address, and I have no additional feedback to provide.

[qinxuye]

LGTM