fix(fetch): add python-only readability fallback

[he-yufeng]

Fixes #4199.

Summary

mcp-server-fetch always called readabilipy with use_readability=True. That path shells out through Node.js, but the server is installed and documented as a Python package. If Node is missing or the Node readability path is slow/misconfigured, HTML fetches can hang until the MCP client times out.

This patch keeps the stronger Node.js readability path when node is available, but avoids making it an undeclared hard runtime requirement:

• auto-detect node before choosing the readability backend
• fall back to readabilipy's Python-only path when Node is unavailable
• add --no-readability-js for hosts that have Node installed but want the Python-only backend
• document the behavior in the fetch README

Validation

• python -m py_compile src/fetch/src/mcp_server_fetch/server.py src/fetch/tests/test_server.py
• src/fetch/.venv/Scripts/python.exe -m pip install -e src/fetch
• src/fetch/.venv/Scripts/python.exe -m pytest tests -q from src/fetch
• src/fetch/.venv/Scripts/python.exe -m py_compile src/mcp_server_fetch/server.py tests/test_server.py from src/fetch
• src/fetch/.venv/Scripts/python.exe -m ruff check src/mcp_server_fetch/server.py tests/test_server.py from src/fetch
• git diff --check origin/main..HEAD