PYTHON-5784 Coverage increase for periodic_executor.py

[aclark4life]

PYTHON-5784

Changes in this PR

Adds test/asynchronous/test_periodic_executor.py (synchro'd to test/test_periodic_executor.py) with unit tests for pymongo/periodic_executor.py, covering both PeriodicExecutor and AsyncPeriodicExecutor:

• join() before open() is a safe no-op.
• Target returning False stops the executor.
• Target raising an exception stops the executor (verified by asserting target is called exactly once).
• skip_sleep() causes the next iteration to run without waiting the full interval.
• wake() interrupts the current sleep and triggers an early run.
• open() after the target returned False restarts the executor.

Test Plan

Added unit tests using asyncio.Event / threading.Event for deterministic synchronization. The exception test swaps threading.excepthook to a no-op so the intentional RuntimeError in the worker thread is not surfaced by pytest's threadexception plugin (no-op for the async executor since no thread is involved).

uv run --extra test python -m pytest test/asynchronous/test_periodic_executor.py test/test_periodic_executor.py -v

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?