feat: add auto refresh on thread listeners (#9105)

This PR is in the scope of multi-agent work. 

An agent (=thread) can now spawn other agents. Those other agents are
not attached to any clients. We need a way to make sure that the clients
are aware of the new threads to look at (for approval for example). This
PR adds a channel to the `ThreadManager` that pushes the ID of those
newly created agents such that the client (here the app-server) can also
subscribe to those ones.

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

@@ -28,7 +28,9 @@ use codex_core::default_client::USER_AGENT_SUFFIX;
 use codex_core::default_client::get_codex_user_agent;
 use codex_core::default_client::set_default_originator;
 use codex_feedback::CodexFeedback;
+use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
+use tokio::sync::broadcast;
 use toml::Value as TomlValue;
 
 pub(crate) struct MessageProcessor {
@@ -159,7 +161,6 @@ impl MessageProcessor {
                     self.outgoing.send_response(request_id, response).await;
 
                     self.initialized = true;
-
                     if !self.config_warnings.is_empty() {
                         for notification in self.config_warnings.drain(..) {
                             self.outgoing
@@ -214,6 +215,19 @@ impl MessageProcessor {
         tracing::info!("<- notification: {:?}", notification);
     }
 
+    pub(crate) fn thread_created_receiver(&self) -> broadcast::Receiver<ThreadId> {
+        self.codex_message_processor.thread_created_receiver()
+    }
+
+    pub(crate) async fn try_attach_thread_listener(&mut self, thread_id: ThreadId) {
+        if !self.initialized {
+            return;
+        }
+        self.codex_message_processor
+            .try_attach_thread_listener(thread_id)
+            .await;
+    }
+
     /// Handle a standalone JSON-RPC response originating from the peer.
     pub(crate) async fn process_response(&mut self, response: JSONRPCResponse) {
         tracing::info!("<- response: {:?}", response);