core[patch]: Support non ASCII characters in tool output if user does…

…n't output string (langchain-ai#26319)

### simple modify
core: add supporting non english character

target issue is langchain-ai#26315 
same issue on langgraph -
langchain-ai/langgraph#1504

libs/core/langchain_core/tools/base.py

@@ -822,7 +822,7 @@ def _is_message_content_block(obj: Any) -> bool:
 
 def _stringify(content: Any) -> str:
     try:
-        return json.dumps(content)
+        return json.dumps(content, ensure_ascii=False)
     except Exception:
         return str(content)