Persist multi-agent runtime metadata (#25721)

Stack split from #25708. Original PR intentionally left open. This
second PR persists multi-agent runtime metadata through thread creation,
rollout recording, and thread storage.

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

@@ -558,6 +558,7 @@ impl Session {
                                     text: session_configuration.base_instructions.clone(),
                                 },
                                 dynamic_tools: session_configuration.dynamic_tools.clone(),
+                                multi_agent_version: None,
                                 metadata: ThreadPersistenceMetadata {
                                     cwd: Some(config.cwd.to_path_buf()),
                                     model_provider: config.model_provider_id.clone(),

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

@@ -3461,6 +3461,7 @@ async fn attach_thread_persistence(session: &mut Session) -> PathBuf {
             thread_source: None,
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
+            multi_agent_version: None,
             metadata: ThreadPersistenceMetadata {
                 cwd: Some(config.cwd.to_path_buf()),
                 model_provider: config.model_provider_id.clone(),
@@ -5957,6 +5958,7 @@ async fn shutdown_complete_does_not_append_to_thread_store_after_shutdown() {
             thread_source: None,
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
+            multi_agent_version: None,
             metadata: ThreadPersistenceMetadata {
                 cwd: Some(config.cwd.to_path_buf()),
                 model_provider: config.model_provider_id.clone(),