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/core/src/codex_thread.rs

@@ -858,6 +858,11 @@ impl CodexThread {
         self.io.agent_status().await
     }
 
+    /// Returns whether the runtime has published its shutdown status.
+    pub fn has_shutdown_status(&self) -> bool {
+        matches!(*self.io.agent_status.borrow(), AgentStatus::Shutdown)
+    }
+
     pub async fn list_background_terminals(&self) -> Vec<BackgroundTerminalInfo> {
         self.session.list_background_terminals().await
     }