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/core/src/tasks/mod.rs

@@ -766,11 +766,13 @@ impl Session {
                 .turn_timing_state
                 .time_to_first_token_ms()
                 .await;
+            let error = turn_context.terminal_error.lock().await.clone();
             self.emit_turn_stop_lifecycle(turn_context.extension_data.as_ref())
                 .await;
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: turn_context.sub_id.clone(),
                 last_agent_message,
+                error,
                 started_at,
                 completed_at,
                 duration_ms,