PR #13021 fixed serial starvation by adding ThreadPoolExecutor to tick(),
but kept as_completed(timeout=600) which still blocks the ticker thread
until the slowest job finishes. This causes the same starvation pattern:
when one job runs long (15+ min), other jobs' next_run_at expires past the
grace window and they get perpetually fast-forwarded instead of running.
This PR decouples dispatch from completion:
- Persistent ThreadPoolExecutor (reused across ticks, no auto-join)
- Fire-and-forget dispatch: tick submits and returns immediately
- Running-job guard: prevents re-dispatching active jobs
- sync parameter: defaults to True (backward compatible), callers opt
into sync=False for non-blocking behavior
- atexit shutdown handler for clean pool teardown
- gateway/run.py: production ticker opts into sync=False
Refs #33315 (complementary — that issue's PRs fix grace handling in
jobs.py; this PR prevents the grace from expiring in the first place)
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