Make extension lifecycle hooks async (#23291)

## Why

Extension lifecycle hooks sit on the host/extension boundary, but the
current trait surface only allows synchronous callbacks. That forces
extensions that need to seed, rehydrate, observe, or flush
extension-owned state during thread and turn transitions to either block
inside the callback or move async work into separate host plumbing.

This PR makes those lifecycle callbacks awaitable so extension
implementations can perform async work directly at the lifecycle point
where the host already has the relevant session, thread, or turn stores
available.

## What changed

- Makes `ThreadLifecycleContributor` and `TurnLifecycleContributor`
async in `codex-extension-api`.
- Awaits thread start/resume/stop and turn start/stop/abort lifecycle
callbacks from `codex-core`.
- Updates the guardian and memories extensions to implement the async
lifecycle trait surface.
- Updates the existing lifecycle tests to use async contributor
implementations.
- Adds `async-trait` to the crates that now expose or implement these
async object-safe lifecycle traits.

## Testing

- Existing `codex-core` lifecycle tests were updated to cover async
implementations for thread stop and turn abort ordering.

codex-rs/core/src/session/handlers.rs

@@ -638,12 +638,14 @@ async fn shutdown_session_runtime(sess: &Arc<Session>) {
     sess.guardian_review_session.shutdown().await;
 }
 
-fn emit_thread_stop_lifecycle(sess: &Session) {
+async fn emit_thread_stop_lifecycle(sess: &Session) {
     for contributor in sess.services.extensions.thread_lifecycle_contributors() {
-        contributor.on_thread_stop(codex_extension_api::ThreadStopInput {
-            session_store: &sess.services.session_extension_data,
-            thread_store: &sess.services.thread_extension_data,
-        });
+        contributor
+            .on_thread_stop(codex_extension_api::ThreadStopInput {
+                session_store: &sess.services.session_extension_data,
+                thread_store: &sess.services.thread_extension_data,
+            })
+            .await;
     }
 }
 
@@ -662,7 +664,7 @@ pub async fn shutdown(sess: &Arc<Session>, sub_id: String) -> bool {
         &[],
     );
 
-    emit_thread_stop_lifecycle(sess.as_ref());
+    emit_thread_stop_lifecycle(sess.as_ref()).await;
 
     // Gracefully flush and shutdown thread persistence on session end so tests
     // that inspect durable state do not race with the background writer.
@@ -917,7 +919,7 @@ pub(super) async fn submission_loop(
     // explicit shutdown op, still run session teardown.
     if !shutdown_received {
         shutdown_session_runtime(&sess).await;
-        emit_thread_stop_lifecycle(sess.as_ref());
+        emit_thread_stop_lifecycle(sess.as_ref()).await;
     }
     debug!("Agent loop exited");
 }

codex-rs/core/src/session/session.rs

@@ -877,7 +877,7 @@ impl Session {
                     config: config.as_ref(),
                     session_store: &session_extension_data,
                     thread_store: &thread_extension_data,
-                });
+                }).await;
             }
 
             let services = SessionServices {

codex-rs/core/src/session/tests.rs

@@ -5683,8 +5683,9 @@ async fn submission_loop_channel_close_emits_thread_stop_lifecycle() {
         expected_thread_id: ThreadId,
     }
 
+    #[async_trait::async_trait]
     impl codex_extension_api::ThreadLifecycleContributor<crate::config::Config> for ThreadStopRecorder {
-        fn on_thread_stop(&self, input: codex_extension_api::ThreadStopInput<'_>) {
+        async fn on_thread_stop(&self, input: codex_extension_api::ThreadStopInput<'_>) {
             assert_eq!(
                 self.expected_thread_id.to_string(),
                 input.thread_store.level_id()
@@ -5728,8 +5729,9 @@ async fn submission_loop_channel_close_aborts_active_turn_before_thread_stop_lif
         expected_turn_id: String,
     }
 
+    #[async_trait::async_trait]
     impl codex_extension_api::ThreadLifecycleContributor<crate::config::Config> for LifecycleRecorder {
-        fn on_thread_stop(&self, input: codex_extension_api::ThreadStopInput<'_>) {
+        async fn on_thread_stop(&self, input: codex_extension_api::ThreadStopInput<'_>) {
             assert_eq!(
                 self.expected_thread_id.to_string(),
                 input.thread_store.level_id()
@@ -5741,8 +5743,9 @@ async fn submission_loop_channel_close_aborts_active_turn_before_thread_stop_lif
         }
     }
 
+    #[async_trait::async_trait]
     impl codex_extension_api::TurnLifecycleContributor for LifecycleRecorder {
-        fn on_turn_abort(&self, input: codex_extension_api::TurnAbortInput<'_>) {
+        async fn on_turn_abort(&self, input: codex_extension_api::TurnAbortInput<'_>) {
             assert_eq!(
                 self.expected_thread_id.to_string(),
                 input.thread_store.level_id()