run_one_job's dispatch-failure handler already routes an
_ExternalWorkerPostHandoffError through bookkeeping only (no incident, no
ping), but the recorded error still read "Restart-safe cron worker dispatch
failed", which is misleading in last_error / executions.db for a worker that
was spawned and may have run. Compute post_handoff first and label it
"Restart-safe cron worker failed after handoff: ..."; the pre-handoff prefix
is unchanged.