protected CachedSubscriptionPollResponse peekFirst() {
    return responses.isEmpty() ? null : responses.getFirst();
  }
The {@code SubscriptionEventExtendableResponse} class represents a subscription event response
that can dynamically change as new responses are fetched. It maintains a list of {@link
CachedSubscriptionPollResponse} objects and provides methods for managing and serializing these
responses.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/subscription/event/response/SubscriptionEventExtendableResponse.java