anthropic: `FilesystemClaudeTextEditorMiddleware` reads and writes files with the locale encoding so non ascii content breaks on non utf8 systems

### 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-typesafe - [ ] langchain-xai - [ ] Other / not sure / general ### Related Issues / PRs #33510 added the filesystem middleware #40588 and #40664 are the same kind of problem (locale encoding) but in the test suite not in the library ### Reproduction Steps / Example Code (Python) ```python # run with a non utf8 locale, on windows just use a cp1252 or cp936 system # on mac or linux you can simulate it with # LC_ALL=C LANG=C PYTHONCOERCECLOCALE=0 PYTHONUTF8=0 python repro.py import locale, tempfile from pathlib import Path from langchain_anthropic.middleware import FilesystemClaudeTextEditorMiddleware print("locale encoding:", locale.getencoding()) root = tempfile.mkdtemp() mw = FilesystemClaudeTextEditorMiddleware(root_path=root) # 1. the model creates a file with some non ascii text try: mw._handle_create({"path": "/notes.md", "file_text": "# Café ☕\nnaïve 🙂"}, "tc1") print("…