Funnel session finalization through AIAgent.close() — the single terminal
path every agent (CLI, gateway, subagent, cron) funnels through — so finished
agents stop leaving rows with ended_at IS NULL. The biggest leak source was
delegate_task subagent + background-review forks whose close() never ended
their row.
end_session() is first-reason-wins and no-ops on an already-ended row, so a
'compression'/'cron_complete'/'cli_close' reason set by an earlier terminal
path is never clobbered. /resume already calls reopen_session(), so
finalizing-on-close does not break resumability.
Temporary helper agents that rotate/share the session forward (manual
compression, gateway session-hygiene) opt out via _end_session_on_close=False.
Also stop the long-running gateway heartbeat once the executor is done or the
session slot is rebound to a different agent, preventing a stale
'running: delegate_task' bubble from outliving its run.
Closes#12029.
b17180d950