fix: set wait_for_completion=True always in delete_all_documents

Both sync and async delete_all_documents need to wait for completion
so documents are actually deleted before the function returns

Author: GunaPalanivel

integrations/opensearch/src/haystack_integrations/document_stores/opensearch/document_store.py

@@ -600,7 +600,7 @@ def _prepare_delete_all_request(self, *, is_async: bool, refresh: bool) -> dict[
         return {
             "index": self._index,
             "body": {"query": {"match_all": {}}},  # Delete all documents
-            "wait_for_completion": not is_async,  # `True` for sync calls only
+            "wait_for_completion": True,  # Always wait to ensure documents are deleted before returning
             "refresh": refresh,
         }