feat: guardian as an extension (contributors part) (#22216)

Part 1 of guardian as extension. This bind all the logic to spawn
another agent from an extension and it adds `ThreadId` in the start
thread collaborator

codex-rs/core/src/session/session.rs

@@ -815,6 +815,7 @@ impl Session {
             let thread_extension_data = codex_extension_api::ExtensionData::new();
             for contributor in extensions.thread_start_contributors() {
                 contributor.contribute(
+                    thread_id,
                     config.as_ref(),
                     &session_extension_data,
                     &thread_extension_data,