Defense-in-depth alongside the interpreter-shutdown guard: run_job closed
the cron agent's async resources (agent.close + cleanup_stale_async_clients)
in its finally block BEFORE run_one_job called _deliver_result, so a live
delivery could race a torn-down async client. run_job now accepts an optional
defer_agent_teardown holder; when set it hands the live agent back instead of
closing it, and run_one_job tears it down (via the extracted _teardown_cron_agent
helper) only AFTER delivery — in a finally so a failed run never leaks. Default
path (holder=None) is unchanged, so every existing caller keeps inline teardown.
Reorder approach based on #58777 by @LavyaTandel; reworked to keep a single
delivery site in run_one_job and add regression coverage.
Co-authored-by: LavyaTandel <lavya@loom.local>
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