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/message_processor.rs

@@ -456,6 +456,12 @@ impl MessageProcessor {
         self.codex_message_processor.drain_background_tasks().await;
     }
 
+    pub(crate) async fn clear_all_thread_listeners(&self) {
+        self.codex_message_processor
+            .clear_all_thread_listeners()
+            .await;
+    }
+
     pub(crate) async fn shutdown_threads(&self) {
         self.codex_message_processor.shutdown_threads().await;
     }