Support asynchronous command hooks (#37533)

## Why

Hook configurations can mark command handlers as asynchronous, but Codex previously skipped those handlers outside `SessionEnd`.

## What changed

- Run asynchronous command hooks in the background with a per-session concurrency limit, while keeping `SessionEnd` hooks synchronous.
- Prevent asynchronous hooks from blocking, stopping, rewriting, or otherwise controlling the operation that launched them.
- Deliver warnings and additional context at safe turn boundaries: inject results into an active turn after sampling, or buffer them ahead of the next user prompt when the session is idle.
- Preserve in-flight hooks across configuration reloads, scope spilled output to the thread, and abort outstanding work during session shutdown.

## Testing

Add unit and integration coverage for background scheduling, concurrency, output parsing, active and idle result delivery, configuration reloads, and shutdown cleanup.

GitOrigin-RevId: 8094552e4afe7b47b09a61bb575bb20f4e491d8d

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

@@ -21,6 +21,7 @@ use codex_protocol::config_types::ServiceTier;
 use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::permissions::FileSystemPath;
 use codex_protocol::permissions::FileSystemSpecialPath;
+use codex_protocol::protocol::HookCompletedEvent;
 use codex_protocol::protocol::MultiAgentVersion;
 use codex_protocol::protocol::ThreadHistoryMode;
 use codex_protocol::protocol::ThreadSource;
@@ -55,6 +56,7 @@ pub(crate) struct Session {
     pub(super) mcp_prewarm_task: std::sync::Mutex<Option<JoinHandle<()>>>,
     pub(crate) conversation: Arc<RealtimeConversationManager>,
     pub(crate) active_turn: Mutex<Option<ActiveTurn>>,
+    pub(crate) async_hook_results: async_channel::Receiver<HookCompletedEvent>,
     pub(crate) pending_user_message_admissions:
         crate::user_message_admission::PendingUserMessageAdmissions,
     pub(crate) input_queue: InputQueue,
@@ -1169,12 +1171,13 @@ impl Session {
                     (None, None)
                 };
 
-            let hooks = build_hooks_for_config(
+            let hooks_config = build_hooks_config(
                 &config,
                 plugins_manager.as_ref(),
                 resolved_environments.single_local_environment(),
             )
             .await;
+            let (hooks, async_hook_results) = Hooks::new(hooks_config, thread_id);
             for warning in hooks.startup_warnings() {
                 post_session_configured_events.push(Event {
                     id: INITIAL_SUBMIT_ID.to_owned(),
@@ -1320,6 +1323,7 @@ impl Session {
                 mcp_prewarm_task: std::sync::Mutex::new(None),
                 conversation: Arc::new(RealtimeConversationManager::new()),
                 active_turn: Mutex::new(None),
+                async_hook_results,
                 pending_user_message_admissions: Default::default(),
                 input_queue: InputQueue::new(),
                 guardian_review_session: GuardianReviewSessionManager::default(),