### Summary
Release live thread persistence when a session ends because its
submission channel closes. This prevents a later same-process resume
from failing with `thread ... already has a live local writer`.
### Details
The issue is in the `codex-core` session teardown path used by Codex
hosts, rather than in Managed Agents API or exec-server itself.
Explicit shutdown already closes the `LiveThread`, which releases the
process-scoped writer held by `LocalThreadStore`. The
submission-channel-close fallback ran runtime and extension teardown but
skipped that persistence shutdown, leaving the thread ID registered as
having a live writer.
This change:
- closes the `LiveThread` on the channel-close fallback path;
- preserves the existing teardown order used by explicit shutdowns;
- extends the lifecycle regression test to assert that the thread store
receives `shutdown_thread`.
Context: [original
report](https://openai.slack.com/archives/C0B4NBHQGTV/p1782136364948039),
[recent occurrence
1](https://openai.slack.com/archives/C0B4NBHQGTV/p1782434817895839?thread_ts=1782136364.948039&cid=C0B4NBHQGTV),
[recent occurrence
2](https://openai.slack.com/archives/C0B4NBHQGTV/p1782335107474429?thread_ts=1782136364.948039&cid=C0B4NBHQGTV)
### Testing
- `just test -p codex-core
submission_loop_channel_close_runs_full_thread_teardown`
- `just test -p codex-core --lib` (1,989 passed; 3 skipped)
- `just fix -p codex-core`
- `just fmt`
- Native code review: no findings
I also attempted `just test -p codex-core`. The new regression passed;
79 unrelated integration tests failed in the local harness, primarily
because helper binaries such as `test_stdio_server` were unavailable,
plus local proxy/shell timing failures.
Abdulrahman Alfozan
c55ce3b51b