Track root turns across delegated Codex requests (#38232)

## Why

`parent_turn_id` identifies the immediate caller of a nested request, but not the
top-level turn that initiated a multi-level chain of work.

## What changed

- Add `root_turn_id` to turn state, submissions, and Responses API metadata.
- Propagate the root through spawned agents, delegated work, review sessions, and
  turn-triggering inter-agent messages.
- Omit the root when background work has no user turn or combined input makes the
  root ambiguous.

## Testing

Added coverage for nested agent resumes, reviews, mailbox delivery, steered input,
and reserved metadata handling.

GitOrigin-RevId: 60057db2ee81f4efefa946a9f63c74ff98b93929

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

@@ -830,8 +830,10 @@ impl Session {
 impl SessionIo {
     /// Submit the `op` wrapped in a `Submission` with a unique ID.
     pub(crate) async fn submit(&self, op: Op) -> CodexResult<String> {
-        self.submit_with_trace(op, /*trace*/ None, /*parent_turn_id*/ None)
-            .await
+        self.submit_with_trace(
+            op, /*trace*/ None, /*parent_turn_id*/ None, /*root_turn_id*/ None,
+        )
+        .await
     }
 
     pub(crate) async fn submit_with_trace(
@@ -839,6 +841,7 @@ impl SessionIo {
         op: Op,
         trace: Option<W3cTraceContext>,
         parent_turn_id: Option<String>,
+        root_turn_id: Option<String>,
     ) -> CodexResult<String> {
         let id = new_submission_id();
         let sub = Submission {
@@ -847,6 +850,7 @@ impl SessionIo {
             client_user_message_id: None,
             trace,
             parent_turn_id,
+            root_turn_id,
         };
         self.submit_with_id(sub).await?;
         Ok(id)
@@ -866,6 +870,7 @@ impl SessionIo {
             client_user_message_id,
             trace,
             parent_turn_id: None,
+            root_turn_id: None,
         };
         self.submit_with_id(sub).await?;
         Ok(id)
@@ -1259,6 +1264,7 @@ impl Session {
             },
             /*client_user_message_id*/ None,
             /*parent_turn_id*/ None,
+            /*root_turn_id*/ None,
         )
         .await;
     }
@@ -2046,6 +2052,7 @@ impl Session {
                 communication,
                 context,
                 /*parent_turn_id*/ None,
+                /*root_turn_id*/ None,
             )
             .await
         {
@@ -4025,6 +4032,7 @@ impl Session {
         expected_turn_id: Option<&str>,
         client_user_message_id: Option<String>,
         responsesapi_client_metadata: Option<HashMap<String, String>>,
+        incoming_turn_metadata: Option<&TurnMetadataState>,
     ) -> Result<String, SteerInputError> {
         let mut active = self.active_turn.lock().await;
         let Some(active_turn) = active.as_mut() else {
@@ -4084,6 +4092,15 @@ impl Session {
             content: input,
             client_id: client_user_message_id.clone(),
         });
+        if let Some(incoming_turn_metadata) = incoming_turn_metadata
+            && active_task.turn_context.turn_metadata_state.root_turn_id()
+                != incoming_turn_metadata.root_turn_id()
+        {
+            active_task
+                .turn_context
+                .turn_metadata_state
+                .mark_root_turn_ambiguous();
+        }
         self.input_queue
             .extend_pending_input_and_accept_mailbox_delivery_for_turn_state(
                 active_turn.turn_state.as_ref(),