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/app-server/src/codex_message_processor.rs

@@ -3288,6 +3288,7 @@ impl CodexMessageProcessor {
                 summary,
                 final_output_json_schema: output_schema,
                 collaboration_mode: None,
+                personality: None,
             })
             .await;
 
@@ -3415,6 +3416,7 @@ impl CodexMessageProcessor {
                     effort: params.effort.map(Some),
                     summary: params.summary,
                     collaboration_mode: params.collaboration_mode,
+                    personality: None,
                 })
                 .await;
         }