Salvage of #65260's b7d7cfd0e (ported — the PR's close() predates ~4K
commits of teardown-step churn, so the hunk is re-anchored after step
6b rather than cherry-picked).
agent/codex_runtime.py already drops _codex_session on turn crash and
on retirement, but AIAgent.close() — the hard teardown for /new,
/reset, and session expiry — had no owner for it, so the app-server
child process survived until interpreter exit. Long-lived gateways
accumulate one leaked subprocess per ended Codex session.
The attribute is cleared BEFORE close() so a concurrent reader can't
observe a half-closed session and a raising close() can't strand a
stale reference (tested).
Tests extend the author's original lifecycle test with the
raising-close and no-codex-session cases.
JR Razmus