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/review.rs

@@ -121,6 +121,9 @@ pub(super) async fn spawn_review_thread(
         parent_turn_context.network.is_some(),
         auto_review_enabled,
     ));
+    if turn_metadata_state.can_start_root_turn(&session_source) {
+        turn_metadata_state.set_root_turn_id(review_turn_id.clone());
+    }
 
     let extension_data = Arc::new(codex_extension_api::ExtensionData::new(
         review_turn_id.clone(),