fix(TreeDropTargetDelegate): only normalize to 'after' target if valid

[reidbarber]

Closes #8561

We normalize to the 'after' drop target, but previously this could have been one of the dragged items (not a valid drop target). Now, we make sure the normalized target is a valid drop target before using it.

✅ 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:

Try steps from #8561

🧢 Your Project:

[rspbot]

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