The four CI reds from #41225:
- persist_on_release is a background-only terminal modifier: join the
sandbox blocked sets (precedent: heartbeat, 9acd0d33b6) in
_TERMINAL_BLOCKED_PARAMS and the stub-drift tests' mirrors.
- The gateway shutdown sweep passes source="gateway_shutdown" so
persisted jobs are still killed on host exit; the two shutdown tests'
kill_all fakes now accept and assert that kwarg instead of raising
TypeError that _quiet_step silently swallowed.
The execute_code sandbox refuses background/notify modifiers on terminal(); heartbeat implies
notify and rides the same delivery path, so it joins the blocked set (and the stub-drift test's
mirror of it).
