Fix app-server thread listener startup and shutdown races (#57787)

## Why

A thread runtime can shut down while listener setup is awaiting I/O, and a cold resume can start before the previous listener forwards `ShutdownComplete`. Listener replacement must preserve those buffered terminal events.

## What changed

- Coordinate listener installation and connection subscriptions with pending unloads, and reject closing or replaced runtimes.
- Allow a newly created runtime that has already shut down to drain its queued events, while enforcing one event reader per runtime.
- Track shutdown draining with a shared watch channel so cold resume and runtime replacement wait for terminal events to be processed.
- Serialize idle unloading with goal edits and cold resume, and recheck thread state after acquiring the lock.

GitOrigin-RevId: d05e348b0a81bddb2defa963d0cfd3fde08dbd03

codex-rs/app-server/src/message_processor.rs

@@ -866,8 +866,16 @@ impl MessageProcessor {
         thread_id: ThreadId,
         connection_ids: Vec<ConnectionId>,
     ) {
+        let Ok(thread) = self
+            .thread_processor
+            .thread_manager
+            .get_thread(thread_id)
+            .await
+        else {
+            return;
+        };
         self.thread_processor
-            .try_attach_thread_listener(thread_id, connection_ids)
+            .try_attach_thread_listener(thread_id, thread, connection_ids)
             .await;
     }