Phase 4A. Factor tick's per-job closure (_process_job: execute → save →
deliver → mark) into a module-level run_one_job(job, *, adapters, loop,
verbose) so the external Chronos provider's fire_due (Phase 4D) reuses the
IDENTICAL body — no duplicated correctness. tick's _process_job is now a thin
wrapper calling run_one_job; the pool/in-flight-guard/contextvars dispatch
logic is unchanged.
run_one_job fires ONE given job; it does NOT decide due-ness, claim, or compute
next_run (tick advances next_run_at under the file lock; an external provider
claims via the store CAS in Phase 4C). Pure refactor, no behavior change.
TDD: test_run_one_job.py characterizes the sequence through tick() first
(test_tick_process_job_sequence, passed pre-extraction), then unit-tests the
helper directly: success sequence, [SILENT]→skip delivery, empty-response soft
failure (#8585), failed-job-still-delivers, exception→mark-failed.
Verified: tests/cron/ 459 passed (was 453 + 6 new); tick behavior unchanged.
