Distinguish Guardian review threads from subagents (#40221)

## Why

Guardian reviews were reported with the generic `subagent` thread source, making them indistinguishable from other delegated threads in persisted metadata and analytics.

## What changed

- Add the `guardian_review` thread source to the core protocol and app-server schema.
- Assign it to Guardian reviewer sessions and propagate the configured source through turn metadata, rollouts, and analytics.
- Continue treating Guardian reviews like subagents for paginated history and trusted-provenance checks.

## Testing

- Cover protocol round trips, delegate source selection, emitted analytics, request metadata, and persisted rollout metadata.

GitOrigin-RevId: 07480c122715812874c9d3b48bb39fc5c86b2367

codex-rs/core/src/codex_delegate_tests.rs

@@ -229,12 +229,13 @@ async fn guardian_delegates_do_not_inherit_parent_extensions() {
         .services
         .extensions = Arc::new(extensions.build());
 
-    for (subagent_source, expected_thread_starts) in [
+    for (subagent_source, expected_thread_starts, expected_thread_source) in [
         (
             SubAgentSource::Other(crate::guardian::GUARDIAN_REVIEWER_NAME.to_string()),
             0,
+            ThreadSource::GuardianReview,
         ),
-        (SubAgentSource::Review, 1),
+        (SubAgentSource::Review, 1, ThreadSource::Subagent),
     ] {
         let mut config = parent_ctx.config.as_ref().clone();
         config.permissions.approval_policy = Constrained::allow_only(AskForApproval::Never);
@@ -263,6 +264,10 @@ async fn guardian_delegates_do_not_inherit_parent_extensions() {
             expected_thread_starts
         );
         assert_eq!(thread_starts.load(Ordering::SeqCst), expected_thread_starts);
+        assert_eq!(
+            session.thread_config_snapshot().await.thread_source,
+            Some(expected_thread_source)
+        );
         io.shutdown_and_wait()
             .await
             .expect("delegate session should shut down");