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/tests/unit_tests/llms/test_base.py

@@ -30,9 +30,12 @@ def test_openai_model_kwargs() -> None:
     assert llm.model_kwargs == {"foo": "bar"}
 
 
-def test_openai_invalid_model_kwargs() -> None:
-    with pytest.raises(ValueError):
-        OpenAI(model_kwargs={"model_name": "foo"})
+def test_openai_fields_in_model_kwargs() -> None:
+    """Test that for backwards compatibility fields can be passed in as model_kwargs."""
+    llm = OpenAI(model_kwargs={"model_name": "foo"})
+    assert llm.model_name == "foo"
+    llm = OpenAI(model_kwargs={"model": "foo"})
+    assert llm.model_name == "foo"
 
 
 def test_openai_incorrect_field() -> None: