feat: add session_id (#20437)

## Summary

Related to
https://openai.slack.com/archives/C095U48JNL9/p1777537279707449
TLDR:
We update the meaning of session ids and thread ids:
* thread_id stays as now
* session_id become a shared id between every thread under a /root
thread (i.e. every sub-agent share the same session id)

This PR introduces an explicit `SessionId` and threads it through the
protocol/client boundary so `session_id` and `thread_id` can diverge
when they need to, while preserving compatibility for older serialized
`session_configured` events.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/codex_thread.rs

@@ -391,7 +391,7 @@ impl CodexThread {
         self.rollout_path.clone()
     }
 
-    pub(crate) fn session_configured(&self) -> SessionConfiguredEvent {
+    pub fn session_configured(&self) -> SessionConfiguredEvent {
         self.session_configured.clone()
     }