fix(async): await async_add_index_struct in adelete_nodes and adelete_ref_doc

Both async methods were calling the synchronous `add_index_struct`
instead of `async_add_index_struct`, creating inconsistent async/sync
usage within async methods. This can cause race conditions in async
contexts where the event loop expects non-blocking operations.

Author: Ricardo-M-L

llama-index-core/llama_index/core/indices/vector_store/base.py

@@ -380,7 +380,7 @@ async def adelete_nodes(
             for node_id in node_ids:
                 self._index_struct.delete(node_id)
                 await self._docstore.adelete_document(node_id, raise_error=False)
-            self._storage_context.index_store.add_index_struct(self._index_struct)
+            await self._storage_context.index_store.async_add_index_struct(self._index_struct)
 
     def delete_nodes(
         self,
@@ -456,7 +456,7 @@ async def adelete_ref_doc(
 
         await asyncio.gather(*tasks)
 
-        self._storage_context.index_store.add_index_struct(self._index_struct)
+        await self._storage_context.index_store.async_add_index_struct(self._index_struct)
 
     @property
     def ref_doc_info(self) -> Dict[str, RefDocInfo]: