Add acknowledged user message submission to core (#36385)

## What changed

- Add `CodexThread::submit_user_input_and_wait_for_admission` and export the
  `UserMessageAdmission` result through `codex-core-api`.
- Resolve submissions only after they start a new turn or steer the active
  turn, returning the accepting turn ID in either case.
- Return errors for invalid operations, rejected thread settings, and session
  termination instead of leaving admission waiters unresolved.

## Testing

Add integration coverage for concurrent start-and-steer submissions, settings
rejection and recovery, non-user operations, and submission after shutdown.

GitOrigin-RevId: 21f1ede38d399baa9836d61a59b9f9af2dbf83b8

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

@@ -52,6 +52,8 @@ 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) pending_user_message_admissions:
+        crate::user_message_admission::PendingUserMessageAdmissions,
     pub(crate) input_queue: InputQueue,
     pub(crate) guardian_review_session: GuardianReviewSessionManager,
     pub(crate) services: SessionServices,
@@ -1186,6 +1188,7 @@ impl Session {
                 mcp_prewarm_task: std::sync::Mutex::new(None),
                 conversation: Arc::new(RealtimeConversationManager::new()),
                 active_turn: Mutex::new(None),
+                pending_user_message_admissions: Default::default(),
                 input_queue: InputQueue::new(),
                 guardian_review_session: GuardianReviewSessionManager::default(),
                 services,