XAI: Make Responses endpoint default

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [x] langchain-xai ### Feature Description I would like to add support for using the Responses API for ChatXAI, instead of chat completions. This is the [recommended way of interacting with the models](https://docs.x.ai/developers/model-capabilities/text/generate-text). I believe this will be beneficial for agents using xAI models. ### Use Case I want to improve the performance of the Grok models in Deepagents CLI. My interest in these models is that Grok 4.20 is showing good performance, I would say up to par with Sonnet 4.5 or better, but at a much lower price. However, most of the issues I encounter have to do with tool calling. Porting to use the Responses API should be a first step for more robust performance of xAI models with Deepagents. ### Proposed Solution I have already implemented a solution in a fork. https://github.com/jdcfd/langchain/tree/xai-responses It mimics the behavior of the ChatOpenAI by redefining this function: ```python def _use_responses_api(self, payload: dict) -> bool: """Determine whether to use the Responses API (the preferred API for xAI). xAI prefers Responses API for models starting with `grok-4` or `grok-code`. """ if isinstance(self.use_responses_api, bool): return self.use_responses_api model_name = self.model_name or payload.get("model", "") if model_name and (model_name.startswith(("grok-4", "grok-code"))): return True return super()._use_responses_api(payload) ``` Most of the functionality is there fr…