ModelRequest.override(messages=...) rejects list[BaseMessage] due to list invariance

### 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 - [ ] 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 #35429 - same `list` invariance problem for `_InputAgentState.messages` #33732 - `list` invariance in `AgentMiddleware` state typing ### Reproduction Steps / Example Code (Python) ```python from langchain.agents.middleware import before_model, ModelRequest from langchain_core.messages import BaseMessage, AIMessage, HumanMessage @before_model def trim_messages(request: ModelRequest) -> ModelRequest: # Filter messages — result is list[BaseMessage] trimmed: list[BaseMessage] = [ m for m in request.messages if isinstance(m, (AIMessage, HumanMessage)) ] # Pyright error: list[BaseMessage] is not assignable to list[AnyMessage] return request.override(messages=trimmed) ``` ### Error Message and Stack Trace (if applicable) ```shell Argument of type "list[BaseMessage]" cannot be assigned to parameter "m…