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/common/test_codex.rs

@@ -280,6 +280,7 @@ impl TestCodex {
                 effort: None,
                 summary: ReasoningSummary::Auto,
                 collaboration_mode: None,
+                personality: None,
             })
             .await?;