create_agent discards state_schema subclass in inferred return type

### 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) - [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 - #33732 (closed Nov 2025 referencing #33817 as the fix). #33817 **partially** fixed the original request — it introduced `StateT_co` and rewired the `middleware` parameter from `AgentMiddleware[AgentState[ResponseT], ContextT]` to `AgentMiddleware[StateT_co, ContextT]`, but it did **not** update `state_schema` or the return type. The original ask in #33732 was for `state_schema` and the return type to use `StateT` as well; that half was not landed. - #34156 (related: custom state schema doesn't propagate to middleware) - langchain-ai/langgraph#6175 (related upstream discussion) ### Reproduction Steps / Example Code (Python) ```python from typing import Annotated, NotRequired from langchain.agents import AgentState, create_agent from langchain_core.language_models.fake_chat_models import FakeListChatModel class MyState(Agen…