[langchain-deepseek] ChatDeepSeek raises KeyError('messages') with use_responses_api=True

### Submission checklist - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [x] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs I searched existing issues and PRs and did not find a matching report for ChatDeepSeek crashing before any HTTP request when using the Responses API. - #38006 concerns provider labels in stream_events(version="v3"); it does not change Responses payload construction or streaming dispatch. - #38896 is a feature request for ChatOpenRouter Responses API support, not ChatDeepSeek. ### Reproduction Steps / Example Code (Python) ```python from langchain_deepseek import ChatDeepSeek # The dummy key and unreachable URL are intentional: the error happens before HTTP. model = ChatDeepSeek( model="deepseek-v4-flash", api_key="test", base_url="https://example.invalid/v1", use_responses_api=True, ) model.invoke("Reply exactly OK") ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description `ChatDeep…