tests/cron/test_scheduler_provider.py spawned a background ticker thread,
slept a fixed 0.2s, then asserted the loop had called tick()/heartbeat() at
least N times. Under loaded CI the worker thread isn't always scheduled
within that window, so the loop hadn't ticked yet — flaking with 'provider
never called tick()' (assert 0 >= 1).
Add a _wait_until(predicate, timeout) helper and replace all five fixed
time.sleep(0.2) sites with a poll on the actual predicate (calls/beats count
reached). Same contract assertions, no wall-clock dependence.
