feat: ephemeral threads (#9765)

Add ephemeral threads capabilities. Only exposed through the
`app-server` v2

The idea is to disable the rollout recorder for those threads.

codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -85,7 +85,7 @@ fn session_configured_produces_thread_started_event() {
             history_log_id: 0,
             history_entry_count: 0,
             initial_messages: None,
-            rollout_path,
+            rollout_path: Some(rollout_path),
         }),
     );
     let out = ep.collect_thread_events(&ev);