core,integrations[minor]: Dont error on fields in model_kwargs (#27110)

Given the current erroring behavior, every time we've moved a kwarg from
model_kwargs and made it its own field that was a breaking change.
Updating this behavior to support the old instantiations /
serializations.

Assuming build_extra_kwargs was not something that itself is being used
externally and needs to be kept backwards compatible

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

@@ -90,7 +90,7 @@ from langchain_core.utils.pydantic import (
     TypeBaseModel,
     is_basemodel_subclass,
 )
-from langchain_core.utils.utils import build_extra_kwargs, from_env, secret_from_env
+from langchain_core.utils.utils import _build_model_kwargs, from_env, secret_from_env
 from pydantic import BaseModel, ConfigDict, Field, SecretStr, model_validator
 from typing_extensions import Self
 
@@ -477,10 +477,7 @@ class BaseChatOpenAI(BaseChatModel):
     def build_extra(cls, values: Dict[str, Any]) -> Any:
         """Build extra kwargs from additional params that were passed in."""
         all_required_field_names = get_pydantic_field_names(cls)
-        extra = values.get("model_kwargs", {})
-        values["model_kwargs"] = build_extra_kwargs(
-            extra, values, all_required_field_names
-        )
+        values = _build_model_kwargs(values, all_required_field_names)
         return values
 
     @model_validator(mode="after")