adding parent_thread_id in guardian (#17249)

## Summary

This PR adds the parent conversation/session id to the subagent-start
analytics event for Guardian subagents.

Previously, Guardian sessions were emitted as subagent
thread-initialized events, but their `parent_thread_id` was serialized
as `null`. After this change, the `codex_thread_initialized` analytics
event for a Guardian child session includes the parent user conversation
id.

codex-rs/analytics/src/events.rs

@@ -249,7 +249,9 @@ pub(crate) fn subagent_thread_started_event_request(
         thread_source: Some("subagent"),
         initialization_mode: ThreadInitializationMode::New,
         subagent_source: Some(subagent_source_name(&input.subagent_source)),
-        parent_thread_id: subagent_parent_thread_id(&input.subagent_source),
+        parent_thread_id: input
+            .parent_thread_id
+            .or_else(|| subagent_parent_thread_id(&input.subagent_source)),
         created_at: input.created_at,
     };
     ThreadInitializedEvent {