[langchain-deepseek] _get_request_payload drops reasoning_content from prior assistant messages, causing 400 errors on tool calls

### 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 Related to #37174 (same root cause; it was auto-closed due to a template/submission issue, so I am re-reporting it here). Also related to the cross-provider tracking issue #34328. ### Reproduction Steps / Example Code (Python) ```python from langchain_deepseek.chat_models import ChatDeepSeek from langchain_core.messages import AIMessage, HumanMessage model = ChatDeepSeek( model="deepseek-v4-flash", # reasoning-capable model api_key="fake_api_key" ) messages = [ HumanMessage(content="Hello!"), AIMessage( content="Hello, can i help you?", additional_kwargs={ "reasoning_content": "Simple greeting, Just greet back in English casually." } ) ] payload = model._get_request_payload(messages) print(payload) # Actual output: reasoning_content is missing from the assist…