fix: Update Tree drag and drop focus

[chirokas]

Closes

✅ 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:

Open the RAC TreeWithDragAndDrop story, drop "Projects" into the second tree (After the insert, focus should be on the newly inserted item), and attempt to keyboard drag "Projects" again into the second tree. Previously, this caused a bug where focus shifted to the collection, but now it should work correctly.

🧢 Your Project:

[andreaaazo]

hi

[snowystinger]

would you mind commenting the code here?

[snowystinger]

is this just a check that there is a dom node being rendered? can we just set the collection to being focused and then let the selectable item focus itself? I'm assuming not, but I don't understand why.
Is it in case you drop an item inside another?

[chirokas]

In short, if an item does not exist in the collection, it means its parent is collapsed. In this case, we should not focus that item, but rather focus its parent, which is consistent with our previous behavior. This applies not only to the case of dropping an item inside another item, but also to the case of dropping a parent item with children into another tree: the newly inserted item is collapsed by default, so we should focus the top-level parent.

[snowystinger]

Thanks for the PR and updates. This way seems like a good start. I'm still mulling over if we want to add something to the collection or somewhere so that droppable collection doesn't need to know about the structure or the rendered items