fix: resolve issue langchain-ai#29513 - cannot retrieve reasoning_con…

…tent while streaming

Extract reasoning_content in the _convert_delta_to_message_chunk function

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -266,6 +266,10 @@ def _convert_delta_to_message_chunk(
     role = cast(str, _dict.get("role"))
     content = cast(str, _dict.get("content") or "")
     additional_kwargs: Dict = {}
+
+    if _dict.get("reasoning_content"):
+        additional_kwargs["reasoning_content"] = _dict["reasoning_content"]
+
     if _dict.get("function_call"):
         function_call = dict(_dict["function_call"])
         if "name" in function_call and function_call["name"] is None:
@@ -1356,9 +1360,9 @@ def with_structured_output(
 
         if method == "json_schema":
             # Check for Pydantic BaseModel V1
-            if (
-                is_pydantic_schema and issubclass(schema, BaseModelV1)  # type: ignore[arg-type]
-            ):
+            if is_pydantic_schema and issubclass(
+                schema, BaseModelV1
+            ):  # type: ignore[arg-type]
                 warnings.warn(
                     "Received a Pydantic BaseModel V1 schema. This is not supported by "
                     'method="json_schema". Please use method="function_calling" '