Fixed drag and drop within Submenus

[Mayank-Tripathi32]

fixes #68178

What?

Fix drag and drop behavior for navigation submenu blocks. Allow proper nesting and ordering when dragging submenu items.

Why?

Bug #68178: Submenu items move to top of list when dragged. Different types of submenu items break drag functionality.

How?

• Modified isDropTargetValid to validate block parent relationships

Testing Instructions

1. Insert a Navigation block
2. Create multiple submenu items with nested items
3. Drag a nested submenu item - verify it maintains position
4. Select multiple submenu items of different types and drag - verify they stay together
5. Test dragging items between different nesting levels

ScreenCast

Submenu.drag.drop.mp4

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Co-authored-by: Mayank-Tripathi32 <[email protected]>
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