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/protocol/src/protocol.rs

@@ -184,6 +184,8 @@ pub struct Submission {
     pub trace: Option<W3cTraceContext>,
     /// Core-provided ID of the parent turn that directly initiated this submission.
     pub parent_turn_id: Option<String>,
+    /// Core-provided ID of the top-level turn that causally initiated this submission.
+    pub root_turn_id: Option<String>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]