fix(submenus): restore safe area behavior when submenu mounts

[reidbarber]

Closes #9641

Looks like this broke in a0d9474 when we removed onResize as a dependency of the effect in useResizeObserver. It was only working before because we were passing a new function every render, so it happened to correct itself.

This PR updates the ref once the submenu is open, so the ref is no longer null and we can measure the submenu rect.

For test coverage, we'll probably need to include a real browser test later (in #9516).

This PR includes the changes from #9639 so we can test the hover behavior, but that should get merged soon.

✅ 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:

Test the safe area hover behavior in a RAC Submenu story.

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs