fix(fetch): avoid Node readability backend by default

[go165]

Summary

• Fixes mcp-server-fetch hard-codes use_readability=True which requires Node.js (undeclared); fails silently when Node is unavailable or misconfigured #4199
• Default the fetch server HTML simplifier to readabilipy's Python backend with use_readability=False
• Keep the existing error marker behavior when simplified content is blank
• Update the fetch README to state the default simplifier does not require Node.js

Verification

• /tmp/mcp-fetch-test-venv/bin/python -m pytest src/fetch/tests/test_server.py
• /tmp/mcp-fetch-test-venv/bin/python -m compileall -q src/fetch/src/mcp_server_fetch src/fetch/tests
• /tmp/mcp-fetch-test-venv/bin/python -m ruff check src/fetch/src/mcp_server_fetch/server.py src/fetch/tests/test_server.py
• git diff --check