Propagate context vars in all classes/methods

- Any direct usage of ThreadPoolExecutor or asyncio.run_in_executor needs manual handling of context vars

libs/core/langchain_core/documents/transformers.py

@@ -1,10 +1,10 @@
 from __future__ import annotations
 
-import asyncio
 from abc import ABC, abstractmethod
-from functools import partial
 from typing import TYPE_CHECKING, Any, Sequence
 
+from langchain_core.runnables.config import run_in_executor
+
 if TYPE_CHECKING:
     from langchain_core.documents import Document
 
@@ -69,6 +69,6 @@ class BaseDocumentTransformer(ABC):
         Returns:
             A list of transformed Documents.
         """
-        return await asyncio.get_running_loop().run_in_executor(
-            None, partial(self.transform_documents, **kwargs), documents
+        return await run_in_executor(
+            None, self.transform_documents, documents, **kwargs
         )