Fix editor lifecycle bugs in EmailEditor

[farhan523]

Summary

Two small bug fixes in the React wrapper's lifecycle handling:

1. Editor was never destroyed on unmount

The unmount cleanup in EmailEditor.tsx closed over editor from the first render, which is always null:

useEffect(() => {
  return () => {
    editor?.destroy(); // editor is always null here
  };
}, []);

So destroy() was a silent no-op and every unmounted editor instance leaked (related: #272). Fixed by tracking the latest instance in a ref. Added a regression test that mounts/unmounts the editor with a mocked embed script and asserts destroy() is called — it fails on the old code and passes with the fix.

2. Broken effect dependency for event-listener props

The event-listener effect depended on Object.keys(methodProps).join(','), but methodProps is an array, so this yields index strings ("0,1,2") rather than prop names. Swapping one on* prop for another (same count) was invisible to React. Changed to methodProps.join(','), which is clearly what was intended.

Testing

• tsdx test — passes, including two new regression tests:
  • unmount destroys the editor (fails without fix 1)
  • unmounting before the embed script finishes loading creates no editor and doesn't throw (covers the scenario from Uncaught Error when quickly mounted and unmounted #272)
• tsdx build — compiles cleanly
• tsdx lint — no new problems introduced (one pre-existing warning removed)