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/exec/src/lib.rs

@@ -455,6 +455,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
                     summary: default_summary,
                     final_output_json_schema: output_schema,
                     collaboration_mode: None,
+                    personality: None,
                 })
                 .await?;
             info!("Sent prompt with event ID: {task_id}");