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/tests/suite/model_switching.rs

@@ -20,6 +20,8 @@ use codex_protocol::openai_models::default_input_modalities;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
+use codex_protocol::protocol::RolloutItem;
+use codex_protocol::protocol::RolloutLine;
 use codex_protocol::user_input::UserInput;
 use core_test_support::responses::ev_completed_with_tokens;
 use core_test_support::responses::ev_image_generation_call;
@@ -179,6 +181,31 @@ async fn model_change_appends_model_instructions_developer_message() -> Result<(
         "expected model switch preamble, got: {model_switch_text:?}"
     );
 
+    test.codex.ensure_rollout_materialized().await;
+    test.codex.flush_rollout().await?;
+    let rollout_path = test.codex.rollout_path().expect("rollout path");
+    let model_states = std::fs::read_to_string(rollout_path)?
+        .lines()
+        .map(serde_json::from_str::<RolloutLine>)
+        .collect::<serde_json::Result<Vec<_>>>()?
+        .into_iter()
+        .filter_map(|line| match line.item {
+            RolloutItem::WorldState(item) => item
+                .state
+                .get("model")
+                .and_then(serde_json::Value::as_str)
+                .map(str::to_string),
+            _ => None,
+        })
+        .collect::<Vec<_>>();
+    assert_eq!(
+        model_states,
+        vec![
+            test.session_configured.model.clone(),
+            next_model.to_string()
+        ]
+    );
+
     Ok(())
 }