fix(fetch): add python-only readability fallback

[he-yufeng]

Description

Make the fetch server use the optional Node.js readability path only when node is available on PATH, and add --no-readability-js for hosts that need to force the Python-only readabilipy simplifier.

This keeps the existing robust simplifier when Node.js is installed, but avoids making a supposedly optional runtime dependency part of the default execution path.

Publishing Your Server

This changes an existing server and does not add a new server to the registry.

Server Details

• Server: fetch
• Changes to: HTML simplification path, CLI configuration, README, tests

Motivation and Context

Fixes #4199.

The README describes Node.js as optional for mcp-server-fetch, but the server always called readabilipy with use_readability=True. In environments without a usable Node.js runtime, that can make HTML fetching fail or hang in a path that should have a Python-only fallback.

The new default is:

• use the Node.js readability simplifier when node is present
• otherwise use readabilipy's Python-only simplifier
• allow explicit opt-out with --no-readability-js

How Has This Been Tested?

From src/fetch:

uv run pytest -q
uv run ruff check .
uv run ruff format --check .
uv run pyright
python -m py_compile src\mcp_server_fetch\server.py src\mcp_server_fetch\__init__.py tests\test_server.py
git diff --check

Breaking Changes

No breaking changes expected. Existing users with Node.js on PATH keep the current readability-js path by default.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

The flag is intentionally opt-out instead of changing users with Node.js installed to the simpler path. The automatic fallback only applies when Node.js is absent.