fix: tui freeze when sub-agents are present (#14816)

The issue was due to a circular `Drop` schema where the embedded
app-server wait for some listeners that wait for this app-server
them-selves.

The fix is an explicit cleaning

**Repro:**
* Start codex
* Ask it to spawn a sub-agent
* Close Codex
* It takes 5s to exit

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

@@ -1921,6 +1921,10 @@ impl CodexMessageProcessor {
         }
     }
 
+    pub(crate) async fn clear_all_thread_listeners(&self) {
+        self.thread_state_manager.clear_all_listeners().await;
+    }
+
     pub(crate) async fn shutdown_threads(&self) {
         let report = self
             .thread_manager