Make the app-server thread unload delay configurable (#42320)

## What changed

- Add the top-level `thread_unload_delay_secs` configuration key for the
  app-server, with a 60-second default instead of the previous 30-minute
  fixed delay.
- Allow zero-delay unloading and reject values that cannot fit in a
  monotonic-clock deadline.
- Reset the unload countdown when a thread gains a subscriber or becomes
  active, and preserve active turns even when the configured delay is zero.
- Document that unloading ephemeral threads discards their in-memory state.

## Testing

- Cover configuration parsing, defaults, overrides, and overflow rejection.
- Cover unsubscribe, resubscribe, delayed and immediate unloading,
  notifications, persisted-thread resume, and WebSocket disconnect behavior.

GitOrigin-RevId: edd46f6b49bbdafbf606bff74378e8f569e95977

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

@@ -1069,6 +1069,7 @@ impl ThreadRequestProcessor {
             thread_list_state_permit: self.thread_list_state_permit.clone(),
             fallback_model_provider: self.config.model_provider_id.clone(),
             codex_home: self.config.codex_home.to_path_buf(),
+            thread_unload_delay: self.config.thread_unload_delay,
             skills_watcher: Arc::clone(&self.skills_watcher),
             turn_cost_worker: self.turn_cost_worker.clone(),
         }
@@ -1200,6 +1201,7 @@ impl ThreadRequestProcessor {
             thread_list_state_permit: self.thread_list_state_permit.clone(),
             fallback_model_provider: self.config.model_provider_id.clone(),
             codex_home: self.config.codex_home.to_path_buf(),
+            thread_unload_delay: self.config.thread_unload_delay,
             skills_watcher: Arc::clone(&self.skills_watcher),
             turn_cost_worker: self.turn_cost_worker.clone(),
         };