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/codex_delegate_tests.rs

@@ -136,6 +136,7 @@ async fn forward_ops_preserves_submission_trace_context() {
             tracestate: Some("vendor=state".to_string()),
         }),
         parent_turn_id: Some("parent-turn".to_string()),
+        root_turn_id: Some("root-turn".to_string()),
     };
     tx_ops.send(submission).await.unwrap();
     drop(tx_ops);