Fix timer cleanup on ComboBox unmount

[ryo-manba]

Closes #2154

The approach aligns with best practices for managing side effects in React, as illustrated in the official documentation.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

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[snowystinger]

This will run every time [isLoading, showLoading, inputValue] changes, which means we may delay certain timeouts.

Instead, this cleanup should be in its own useEffect, which runs on unmount only.

  useEffect(() => {
    return () => {
      clearTimeout(timeout.current);
      timeout.current = null;
    };
  }, []);

The addition of the timeout.current = null is needed to compensate for React StrictMode, otherwise when the second mount happens, the timer won't be set again

[ryo-manba]

Ah, that's a good point. Thanks for the follow-up!