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/shell_snapshot.rs

@@ -101,6 +101,7 @@ async fn run_snapshot_command(command: &str) -> Result<SnapshotRun> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -174,6 +175,7 @@ async fn run_shell_command_snapshot(command: &str) -> Result<SnapshotRun> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;
 
@@ -308,6 +310,7 @@ async fn shell_command_snapshot_still_intercepts_apply_patch() -> Result<()> {
             effort: None,
             summary: ReasoningSummary::Auto,
             collaboration_mode: None,
+            personality: None,
         })
         .await?;