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/image_rollout.rs

@@ -128,6 +128,7 @@ async fn copy_paste_local_image_persists_rollout_request_shape() -> anyhow::Resu
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -208,6 +209,7 @@ async fn drag_drop_image_persists_rollout_request_shape() -> anyhow::Result<()>
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;