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/tool_parallelism.rs

@@ -48,6 +48,7 @@ async fn run_turn(test: &TestCodex, prompt: &str) -> anyhow::Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -365,6 +366,7 @@ async fn shell_tools_start_before_response_completed_when_stream_delayed() -> an
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;