fix(react): prevent double resume in React StrictMode

[jacobwisniewski]

Summary

Fixes #9610

This PR adds a ref guard to prevent resumeStream() from being called twice when React StrictMode double-invokes effects during development.

Problem

When using useChat with resume: true, refreshing the page mid-stream causes duplicate stream requests because React StrictMode runs effects twice (mount → unmount → mount). The current implementation has no protection against this:

useEffect(() => {
  if (resume) {
    chatRef.current.resumeStream();
  }
}, [resume, chatRef]);

This leads to:

• Duplicated/repeated text in the UI
• Multiple concurrent stream consumers
• Inconsistent message state
• onData callbacks being called twice per server write

Solution

Add a hasResumedRef that tracks whether resume has already been attempted:

const hasResumedRef = useRef(false);

useEffect(() => {
  if (resume && !hasResumedRef.current) {
    hasResumedRef.current = true;
    chatRef.current.resumeStream();
  }
}, [resume, chatRef]);

Testing

Added a test case that renders the component in <React.StrictMode> and verifies only one resume request is made despite effects running twice.

Checklist

• Code changes
• Test case added
• Follows existing code style