Track model and personality in world state (#35530)

## What changed

- Add model and personality sections to the persisted world-state snapshot.
- Generate model-switch and personality instructions through world-state diffs, including when prior state must be inferred during replay.
- Keep those instruction fragments in standalone developer messages instead of merging them with adjacent context updates.

## Testing

- Cover initial, unchanged, and changed model and personality states.
- Verify that model changes are recorded in rollout world-state items.

GitOrigin-RevId: 497e29cf38cac08e992e546df900f6f8b01120f9

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

@@ -9359,11 +9359,6 @@ async fn record_context_updates_and_set_reference_context_item_persists_baseline
     }
     let rollout_path = attach_thread_persistence(&mut session).await;
 
-    let update_items = session
-        .build_settings_update_items(Some(&previous_context_item), &turn_context)
-        .await;
-    assert_eq!(update_items, Vec::new());
-
     let turn_context = Arc::new(turn_context);
     let step_context = StepContext::for_test(Arc::clone(&turn_context));
     session