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/inject.rs

@@ -123,6 +123,15 @@ impl Session {
             ));
         }
 
+        if has_user_input
+            && turn_context
+                .turn_metadata_state
+                .can_start_root_turn(&turn_context.session_source)
+        {
+            turn_context
+                .turn_metadata_state
+                .set_root_turn_id(turn_context.sub_id.clone());
+        }
         let task_input = if has_user_input {
             self.clear_connector_selection().await;
             for item in &input {