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/community/langchain_community/llms/llamacpp.py

@@ -8,7 +8,7 @@ from langchain_core.callbacks import CallbackManagerForLLMRun
 from langchain_core.language_models.llms import LLM
 from langchain_core.outputs import GenerationChunk
 from langchain_core.utils import get_pydantic_field_names, pre_init
-from langchain_core.utils.utils import build_extra_kwargs
+from langchain_core.utils.utils import _build_model_kwargs
 from pydantic import Field, model_validator
 
 logger = logging.getLogger(__name__)
@@ -199,10 +199,7 @@ class LlamaCpp(LLM):
     def build_model_kwargs(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
 
     @property