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.rs

@@ -4582,6 +4582,7 @@ pub(crate) fn emit_subagent_session_started(
     analytics_events_client: &AnalyticsEventsClient,
     client_metadata: AppServerClientMetadata,
     thread_id: ThreadId,
+    parent_thread_id: Option<ThreadId>,
     thread_config: ThreadConfigSnapshot,
     subagent_source: SubAgentSource,
 ) {
@@ -4599,6 +4600,7 @@ pub(crate) fn emit_subagent_session_started(
         .as_secs();
     analytics_events_client.track_subagent_thread_started(SubAgentThreadStartedInput {
         thread_id: thread_id.to_string(),
+        parent_thread_id: parent_thread_id.map(|thread_id| thread_id.to_string()),
         product_client_id: client_name.clone(),
         client_name,
         client_version,