Persist loaded threads before managed daemon shutdown (#44283)

## Why

Loaded threads, including idle threads whose rollout files are still deferred, need to survive a managed daemon restart. Shutdown must also remain forceable when rollout I/O is blocked.

## What changed

- Add a hidden `--managed-daemon` option for app-server Unix socket transports. After active turns and admitted requests drain, save loaded persistent root threads before exiting.
- Skip ephemeral threads, non-root agent threads, and threads pending unload. Log persistence failures and continue saving other threads.
- Apply shutdown admission checks to thread and turn settings updates, thread deletion, and archiving.
- Keep force signals and daemon shutdown requests responsive during persistence. Return `AppServerExit::Forced` so executables can exit without waiting for runtime teardown.

## Testing

Add integration coverage for resuming active and idle threads after restart, forcing shutdown during active work, and forcing shutdown with a blocked rollout writer. Extend shutdown rejection coverage to deletion and settings updates.

GitOrigin-RevId: 4344e97d39f9f80c5d84c41c17300fd6aa99b4cc

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -1,3 +1,6 @@
+#[path = "daemon_snapshot.rs"]
+mod daemon_snapshot;
+
 use super::persisted_resume_settings::PersistedResumeSettings;
 use super::persisted_resume_settings::latest_persisted_resume_settings;
 use super::thread_enrichment::enrich_loaded_threads;