fix(dnd): prevent repeated useDrop enter/exit events for portal children

[reidbarber]

Closes #9825

Previously, we were removing tracked elements on every bubbled child dragleave, and portal children bubble through React’s tree even though they are not DOM descendants of the drop target. Now we only remove non-contained elements when the drag is leaving the drop target itself.

✅ 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:

Unit test should cover.

🧢 Your Project:

[rspbot]

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• View the React Aria docs

[reidbarber]

Also, I noticed this Webkit issue just got resolved: https://bugs.webkit.org/show_bug.cgi?id=66547

So maybe eventually we can just use e.relatedTarget.