The TestRetryExhaustion tests from PR #223 didn't mock time.sleep/time.time,
causing the retry backoff loops (275s+ total) to run in real time. Tests would
time out instead of running quickly.
Added _make_fast_time_mock() helper that creates a mock time module where
time.time() advances 500s per call (so sleep_end is always in the past) and
time.sleep() is a no-op. Both tests now complete in <1s.