HuggingFace models: "reasoning" field missing in AIMessageChunk

### 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 - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-cli - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [x] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description Some HuggingFace models return a `"reasoning"` field in their response payloads, which contains intermediate reasoning or chain-of-thought explanations. Currently, the LangChain HuggingFace integration does not surface this field when constructing an `AIMessageChunk`. As a result, the reasoning content is lost and cannot be accessed through standard LangChain interfaces. ### Use Case Users relying on models that provide explicit reasoning need access to this field for: * debugging and observability * agent behaviors that depend on intermediate reasoning signals * evaluation or logging pipelines * transparency and tracing tools Without exposing this reasoning information, the HuggingFace integration provides less insight than the model is capable of delivering. ### Proposed Solution Extend the HuggingFace formatter to extract the `"reasoning"` field from the provider response and insert it into `AIMessageChunk.additional_kwargs`, under a key such as `reasonin…