Add logprobs to ChatResult from ChatLiteLLM

[allen-cook]

When requesting logprobs using ChatLiteLLM the results aren't returned as part of the ChatResponse.response_metadata. This MR adds it in

You can reproduce the problem with this python script using fireworks.ai (or any provider that supplies logprobs)

    chat = ChatLiteLLM(
        model="fireworks_ai/accounts/fireworks/models/deepseek-v3p1",
        temperature=0.3,
        # Pass logprobs and top_logprobs through model_kwargs
        model_kwargs={
            "logprobs": 1,
        }
    )
    messages = [
        SystemMessage(content="You are a helpful assistant."),
        HumanMessage(content="What is the capital of France?")
    ]
    response = chat.invoke(messages)
    print(response.response_metadata["logprobs"])