Filesystem Claude middleware rejects allowed nested paths on Windows

### 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 - [x] 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 #40060 is the same report, automatically closed because it was submitted through the API without an Issue Type. This Web Form submission replaces it. #38106 tightened segment-boundary checks for `allowed_prefixes`, but it does not normalize the filesystem-derived virtual path on Windows. GitHub searches for this middleware, Windows, and `allowed_prefixes` found no issue or PR with the same symptom. ### Reproduction Steps / Example Code (Python) ```python from pathlib import Path from tempfile import TemporaryDirectory from langchain_anthropic.middleware import FilesystemClaudeTextEditorMiddleware with TemporaryDirectory() as temp_dir: root = Path(temp_dir) (root / "workspace" / "nested").mkdir(parents=True) middleware = FilesystemClaudeTextEditorMiddleware( root_path=str(root), allowed_prefixes=[…