[Bug]: Missing initial <think> tag in state accumulation when using stream_mode=["messages"] with Reasoning Models (vLLM/MiniMax)

### Checked other resources - [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) - [x] langchain - [x] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] 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 _No response_ ### Reproduction Steps / Example Code (Python) ```python from langchain.agents import create_agent as _create_agent from langchain_openai import ChatOpenAI from langchain.agents.middleware import TodoListMiddleware from langgraph.graph.state import CompiledStateGraph def create_agent() -> CompiledStateGraph: model = ChatOpenAI( model="MiniMax-M2.5", api_key="sk-xxxxx", base_url="https://127.0.0.1:8000/v1", ) middleware = [ TodoListMiddleware(), ] system_prompt = """You are a helpful assistant that helps users accomplish tasks using tools. """ agents = _create_agent( model=model, system_prompt=system_prompt, middleware=middleware, ) return agents if __name__ == "__main__": from langchain_core.messages imp…