Add deterministic BDD feature and step definitions to resolve the
flaky-test detection alert for test_example_flaky_test (issue #1542).
Root cause: the async-job heartbeat step previously relied on a fixed
time.sleep(0.01) that was insufficient on fast CI runners. When two
consecutive datetime.now(UTC) calls returned the same microsecond value
the 'heartbeat updated' assertion failed intermittently.
The busy-wait guard (already present in async_execution_steps.py) is
the correct fix. This commit adds a dedicated feature that:
- Validates the heartbeat timestamp strictly advances after the
busy-wait (the primary test_example_flaky_test scenario)
- Covers rejection of heartbeat recording for queued and completed jobs
- Adds a bounded heartbeat step that asserts the busy-wait terminates
within a wall-clock budget, preventing infinite hangs on broken clocks
ISSUES CLOSED: #1542
