Do not attempt to append after response.completed (#11402)

Completed responses are fully done, and new response must be created.

codex-rs/codex-api/tests/sse_end_to_end.rs

@@ -161,9 +161,11 @@ async fn responses_stream_parses_items_and_completed_end_to_end() -> Result<()>
         ResponseEvent::Completed {
             response_id,
             token_usage,
+            can_append,
         } => {
             assert_eq!(response_id, "resp1");
             assert!(token_usage.is_none());
+            assert!(!can_append);
         }
         other => panic!("unexpected third event: {other:?}"),
     }