Coverage for fix 4.2:
- _cleanup_old_requests evicts entries older than _MAX_REQUEST_AGE
and leaves fresh entries intact; returns the number removed
- Empty map is safe (no error)
- start_cleanup_thread is idempotent (returns False on second call)
- stop_cleanup_thread joins the thread and clears the handle
- The thread actually evicts stale entries on wakeup
- stop signals the thread to exit promptly via the stop event
instead of waiting for the next interval
