fix: hide unmeasured tooltips during exit

[devo-id]

Closes #10496

When a Tooltip starts its exit animation before its position has been calculated, it can remain mounted with the fallback position: fixed; top: 0; left: 0 styles and briefly appear at the top-left of the viewport.

This change hides the tooltip when it is closed and does not have a computed placement:

visibility: !placement && !state.isOpen ? 'hidden' : undefined

Normally positioned tooltips and their exit animations are unchanged.

Pull Request Checklist:

• Included link to corresponding [React Spectrum GitHub Issue](Combination of Toolbar and Tooltip causes glitching on Tooltip #10496).
• 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](https://www.w3.org/WAI/ARIA/apg/)
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at CLAUDE.md.

Test Instructions:

1. Run:
   yarn jest packages/react-aria-components/test/Tooltip.test.js

2. The regression test hides unmeasured tooltips during exit animation should fail on upstream/main and pass with this change.

3. Also verified:

   • yarn jest packages/react-aria-components/test/Toolbar.test.tsx
   • yarn jest packages/react-aria/test/overlays/useOverlayPosition.test.tsx
   • yarn jest packages/react-aria-components/test/Popover.test.js
   • yarn oxlint packages/react-aria-components/src/Tooltip.tsx packages/react-aria-components/test/Tooltip.test.js
   • yarn oxfmt --check packages/react-aria-components/src/Tooltip.tsx packages/react-aria-components/test/Tooltip.test.js

[devo-id]

I investigated the failing s2-docs check locally by running the same build on both my PR branch and upstream/main. Both fail at packages/dev/s2-docs/pages/s2/dnd.mdx:41 because ./DroppableCOMPONENT cannot be resolved. So this appears to be an existing upstream docs build issue, unrelated to this PR.