langchain-typesafe: ModelRouterMiddleware overrides ModelFallbackMiddleware fallback models

### 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 - [x] langchain-typesafe - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs #40556 is an in-flight rewrite of the TypeSafe middleware. At the time of writing, its `ModelRouterMiddleware` also unconditionally overrides the request model, so the same composition problem appears to remain in that implementation. ### Reproduction Steps / Example Code (Python) ```python from typing import Any from unittest.mock import MagicMock, patch from langchain.agents import create_agent from langchain.agents.middleware import ModelFallbackMiddleware from langchain_core.language_models.fake_chat_models import GenericFakeChatModel from langchain_core.messages import AIMessage, HumanMessage from langchain_core.outputs import ChatResult from langchain_typesafe import ChoiceAnswer from langchain_typesafe.experimental.middleware import ( ModelChoice, ModelRouterMiddleware, ) from langcha…