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/facts.rs

@@ -54,6 +54,7 @@ pub struct AppInvocation {
 #[derive(Clone)]
 pub struct SubAgentThreadStartedInput {
     pub thread_id: String,
+    pub parent_thread_id: Option<String>,
     pub product_client_id: String,
     pub client_name: String,
     pub client_version: String,