fix: should trigger request again when throttle is not undefined

[coverlv]

when experimental_throttle is set, occasionally useChat() will not trigger request again, because the messageRef.current is not the newest messages.

reproduction:

const { append } = useChat({
  maxSteps: 5,
  experimental_throttle: 100
});

Root cause:

// Keep the latest messages in a ref.
const messagesRef = useRef<UIMessage[]>(messages || []);
useEffect(() => {
  messagesRef.current = messages || [];
}, [messages]);

// ...

onUpdate({ message, data, replaceLastMessage }) {
   // has latency here
  throttledMutate(
    newMessages,
    false,
  );
});

const messages = messagesRef.current;
if (
  shouldResubmitMessages({
    originalMaxToolInvocationStep: maxStep,
    originalMessageCount: messageCount,
    maxSteps,
    messages,
  })
)// return false;