Include terminal errors in turn completion events (#32280)

## What changed

- Add an optional `error` payload to `TurnCompleteEvent` and omit it when a turn completes without an error.
- Preserve the full terminal `ErrorEvent` through the turn lifecycle so the completion event includes the same error details emitted separately by `EventMsg::Error`.

## Testing

- Extend the stream-error integration test to verify that `TurnCompleteEvent` contains the emitted error and that the next turn can still proceed.

GitOrigin-RevId: 8d32942a44132763076130102d8b89ab356d59d0

codex-rs/thread-store/src/local/mod.rs

@@ -540,6 +540,7 @@ mod tests {
                     turn_id: "turn-1".to_string(),
                     started_at: None,
                     last_agent_message: None,
+                    error: None,
                     completed_at: None,
                     duration_ms: None,
                     time_to_first_token_ms: None,