feat(core) update Personality on turn (#9644)

## Summary
Support updating Personality mid-Thread via UserTurn/OverwriteTurn. This
is explicitly unused by the clients so far, to simplify PRs - app-server
and tui implementations will be follow-ups.

## Testing
- [x] added integration tests

codex-rs/core/tests/suite/client.rs

@@ -907,6 +907,7 @@ async fn user_turn_collaboration_mode_overrides_model_and_effort() -> anyhow::Re
             summary: config.model_reasoning_summary,
             collaboration_mode: Some(collaboration_mode),
             final_output_json_schema: None,
+            personality: None,
         })
         .await?;