Add unfinished root turn suspension (#40038)

## Why

An active root turn needs to be stopped without marking it complete or aborted
before another runtime can recover the same turn ID.

## What changed

- Add `CodexThread::suspend_turn_and_shutdown` and `SuspendTurnOutcome`.
- Flush history, stop the active regular task, close the history writer, and
  shut down the session without recording a terminal turn event.
- Reject suspension when no supported turn is active or the loaded agent
  subtree still contains a live descendant.

## Testing

- Verify that suspension preserves unfinished history and allows the turn to be
  recovered under its original ID after the descendant guard is cleared.

GitOrigin-RevId: 1f9b019d07c51474ec2d991d263bc15cdd4f89ad

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

@@ -394,7 +394,7 @@ pub async fn set_thread_memory_mode(sess: &Arc<Session>, sub_id: String, mode: T
     }
 }
 
-async fn shutdown_session_runtime(sess: &Arc<Session>) {
+pub(super) async fn shutdown_session_runtime(sess: &Arc<Session>) {
     if let Some(startup_prewarm) = sess.take_session_startup_prewarm().await {
         startup_prewarm.abort().await;
     }
@@ -421,7 +421,7 @@ async fn shutdown_session_runtime(sess: &Arc<Session>) {
     crate::hook_runtime::run_session_end_hooks(sess).await;
 }
 
-async fn emit_thread_stop_lifecycle(sess: &Session) {
+pub(super) 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 {
@@ -585,6 +585,19 @@ pub(super) async fn submission_loop(
                     let _ = reply.send(result);
                     false
                 }
+                Op::SuspendTurnAndShutdown { reply } => {
+                    let result =
+                        super::turn_suspension::suspend_turn_and_shutdown(&sess, sub.id.clone())
+                            .await;
+                    // Exit only after history is durable and its writer has closed; an error
+                    // must leave responsibility for the thread with the current worker.
+                    let should_exit = matches!(
+                        &result,
+                        Ok(codex_protocol::turn_input::SuspendTurnOutcome::Suspended { .. })
+                    );
+                    let _ = reply.send(result);
+                    should_exit
+                }
                 Op::ThreadSettings { thread_settings } => {
                     thread_settings::update(&sess, sub.id.clone(), thread_settings).await;
                     false