anthropic, mistral: return model_name in response metadata (#30048)

Took a "census" of models supported by init_chat_model-- of those that
return model names in response metadata, these were the only two that
had it keyed under `"model"` instead of `"model_name"`.

libs/partners/mistralai/tests/integration_tests/test_chat_models.py

@@ -87,6 +87,7 @@ async def test_ainvoke() -> None:
 
     result = await llm.ainvoke("I'm Pickle Rick", config={"tags": ["foo"]})
     assert isinstance(result.content, str)
+    assert "model_name" in result.response_metadata
 
 
 def test_invoke() -> None: