fix(app-server): emit turn/started only when turn actually starts (#13261)

This is a follow-up for https://github.com/openai/codex/pull/13047

## Why
We had a race where `turn/started` could be observed before the thread
had actually transitioned to `Active`. This was because we eagerly
emitted `turn/started` in the request handler for `turn/start` (and
`review/start`).

That was showing up as flaky `thread/resume` tests, but the real issue
was broader: a client could see `turn/started` and still get back an
idle thread immediately afterward.

The first idea was to eagerly call
`thread_watch_manager.note_turn_started(...)` from the `turn/start`
request path. That turns out to be unsafe, because
`submit(Op::UserInput)` only queues work. If a turn starts and completes
quickly, request-path bookkeeping can race with the real lifecycle
events and leave stale running state behind.

**The real fix** is to move `turn/started` to emit only after the turn
_actually_ starts, so we do that by waiting for the
`EventMsg::TurnStarted` notification emitted by codex core. We do this
for both `turn/start` and `review/start`.

I also verified this change is safe for our first-party codex apps -
they don't have any assumptions that `turn/started` is emitted before
the RPC response to `turn/start` (which is correct anyway).

I also removed `single_client_mode` since it isn't really necessary now.

## Testing
- `cargo test -p codex-app-server thread_resume -- --nocapture`
- `cargo test -p codex-app-server
'suite::v2::turn_start::turn_start_emits_notifications_and_accepts_model_override'
-- --exact --nocapture`
- `cargo test -p codex-app-server`

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

@@ -173,7 +173,6 @@ use codex_app_server_protocol::Turn;
 use codex_app_server_protocol::TurnInterruptParams;
 use codex_app_server_protocol::TurnStartParams;
 use codex_app_server_protocol::TurnStartResponse;
-use codex_app_server_protocol::TurnStartedNotification;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::TurnSteerParams;
 use codex_app_server_protocol::TurnSteerResponse;
@@ -375,7 +374,6 @@ pub(crate) struct CodexMessageProcessor {
     outgoing: Arc<OutgoingMessageSender>,
     arg0_paths: Arg0DispatchPaths,
     config: Arc<Config>,
-    single_client_mode: bool,
     cli_overrides: Vec<(String, TomlValue)>,
     cloud_requirements: Arc<RwLock<CloudRequirementsLoader>>,
     active_login: Arc<Mutex<Option<ActiveLogin>>>,
@@ -402,7 +400,6 @@ struct ListenerTaskContext {
     thread_watch_manager: ThreadWatchManager,
     fallback_model_provider: String,
     codex_home: PathBuf,
-    single_client_mode: bool,
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -419,7 +416,6 @@ pub(crate) struct CodexMessageProcessorArgs {
     pub(crate) config: Arc<Config>,
     pub(crate) cli_overrides: Vec<(String, TomlValue)>,
     pub(crate) cloud_requirements: Arc<RwLock<CloudRequirementsLoader>>,
-    pub(crate) single_client_mode: bool,
     pub(crate) feedback: CodexFeedback,
 }
 
@@ -464,7 +460,6 @@ impl CodexMessageProcessor {
             config,
             cli_overrides,
             cloud_requirements,
-            single_client_mode,
             feedback,
         } = args;
         Self {
@@ -473,7 +468,6 @@ impl CodexMessageProcessor {
             outgoing: outgoing.clone(),
             arg0_paths,
             config,
-            single_client_mode,
             cli_overrides,
             cloud_requirements,
             active_login: Arc::new(Mutex::new(None)),
@@ -2082,7 +2076,6 @@ impl CodexMessageProcessor {
             thread_watch_manager: self.thread_watch_manager.clone(),
             fallback_model_provider: self.config.model_provider_id.clone(),
             codex_home: self.config.codex_home.clone(),
-            single_client_mode: self.single_client_mode,
         };
 
         tokio::spawn(async move {
@@ -5910,17 +5903,8 @@ impl CodexMessageProcessor {
                     status: TurnStatus::InProgress,
                 };
 
-                let response = TurnStartResponse { turn: turn.clone() };
+                let response = TurnStartResponse { turn };
                 self.outgoing.send_response(request_id, response).await;
-
-                // Emit v2 turn/started notification.
-                let notif = TurnStartedNotification {
-                    thread_id: params.thread_id,
-                    turn,
-                };
-                self.outgoing
-                    .send_server_notification(ServerNotification::TurnStarted(notif))
-                    .await;
             }
             Err(err) => {
                 let error = JSONRPCErrorError {
@@ -6211,24 +6195,15 @@ impl CodexMessageProcessor {
         &self,
         request_id: &ConnectionRequestId,
         turn: Turn,
-        parent_thread_id: String,
         review_thread_id: String,
     ) {
         let response = ReviewStartResponse {
-            turn: turn.clone(),
+            turn,
             review_thread_id,
         };
         self.outgoing
             .send_response(request_id.clone(), response)
             .await;
-
-        let notif = TurnStartedNotification {
-            thread_id: parent_thread_id,
-            turn,
-        };
-        self.outgoing
-            .send_server_notification(ServerNotification::TurnStarted(notif))
-            .await;
     }
 
     async fn start_inline_review(
@@ -6244,13 +6219,8 @@ impl CodexMessageProcessor {
         match turn_id {
             Ok(turn_id) => {
                 let turn = Self::build_review_turn(turn_id, display_text);
-                self.emit_review_started(
-                    request_id,
-                    turn,
-                    parent_thread_id.clone(),
-                    parent_thread_id,
-                )
-                .await;
+                self.emit_review_started(request_id, turn, parent_thread_id)
+                    .await;
                 Ok(())
             }
             Err(err) => Err(JSONRPCErrorError {
@@ -6364,7 +6334,7 @@ impl CodexMessageProcessor {
 
         let turn = Self::build_review_turn(turn_id, display_text);
         let review_thread_id = thread_id.to_string();
-        self.emit_review_started(request_id, turn, review_thread_id.clone(), review_thread_id)
+        self.emit_review_started(request_id, turn, review_thread_id)
             .await;
 
         Ok(())
@@ -6540,7 +6510,6 @@ impl CodexMessageProcessor {
                 thread_watch_manager: self.thread_watch_manager.clone(),
                 fallback_model_provider: self.config.model_provider_id.clone(),
                 codex_home: self.config.codex_home.clone(),
-                single_client_mode: self.single_client_mode,
             },
             conversation_id,
             connection_id,
@@ -6628,7 +6597,6 @@ impl CodexMessageProcessor {
                 thread_watch_manager: self.thread_watch_manager.clone(),
                 fallback_model_provider: self.config.model_provider_id.clone(),
                 codex_home: self.config.codex_home.clone(),
-                single_client_mode: self.single_client_mode,
             },
             conversation_id,
             conversation,
@@ -6660,7 +6628,6 @@ impl CodexMessageProcessor {
             thread_watch_manager,
             fallback_model_provider,
             codex_home,
-            single_client_mode,
         } = listener_task_context;
         let outgoing_for_task = Arc::clone(&outgoing);
         tokio::spawn(async move {
@@ -6710,9 +6677,7 @@ impl CodexMessageProcessor {
                         );
                         let raw_events_enabled = {
                             let mut thread_state = thread_state.lock().await;
-                            if !single_client_mode {
-                                thread_state.track_current_turn_event(&event.msg);
-                            }
+                            thread_state.track_current_turn_event(&event.msg);
                             thread_state.experimental_raw_events
                         };
                         let subscribed_connection_ids = thread_state_manager