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/core/src/codex_delegate.rs

@@ -104,6 +104,7 @@ pub(crate) async fn run_codex_thread_interactive(
             &parent_session.services.analytics_events_client,
             client_metadata,
             codex.session.conversation_id,
+            Some(parent_session.conversation_id),
             thread_config,
             subagent_source,
         );