fix(dnd): Virtualized Tree DnD fixes 2 (#8550)

* skip non-item nodes in useDndPersistedKeys

* persist last descendant key for 'after' last key in collection

Author: reidbarber

packages/react-aria-components/src/DragAndDrop.tsx

@@ -76,6 +76,7 @@ export function useDndPersistedKeys(selectionManager: MultipleSelectionManager,
     if (dropState.target.dropPosition === 'after') {
       // Normalize to the "before" drop position since we only render those to the DOM.
       let nextKey = dropState.collection.getKeyAfter(dropTargetKey);
+      let lastDescendantKey: Key | null = null;
       if (nextKey != null) {
         let targetLevel = dropState.collection.getItem(dropTargetKey)?.level ?? 0;
         // Skip over any rows that are descendants of the target ("after" position should be after all children)
@@ -85,16 +86,26 @@ export function useDndPersistedKeys(selectionManager: MultipleSelectionManager,
           if (!node) {
             break;
           }
-          // Stop once we find a node at the same level or higher
+          // Skip over non-item nodes (e.g., loaders) since they can't be drop targets.
+          // eslint-disable-next-line max-depth
+          if (node.type !== 'item') {
+            nextKey = dropState.collection.getKeyAfter(nextKey);
+            continue;
+          }
+
+          // Stop once we find an item at the same level or higher
           // eslint-disable-next-line max-depth
           if ((node.level ?? 0) <= targetLevel) {
             break;
           }
+          
+          lastDescendantKey = nextKey;
           nextKey = dropState.collection.getKeyAfter(nextKey);
         }
       }
 
-      dropTargetKey = nextKey ?? dropTargetKey;
+      // If nextKey is null (end of collection), use the last descendant
+      dropTargetKey = nextKey ?? lastDescendantKey ?? dropTargetKey;
     }
   }