fix(fetch): fall back when readability extraction fails

[kgarg2468]

Summary

• let fetch HTML extraction fall back to readabilipy without Readability.js when readability fails
• add a --no-readability flag for environments without Node/Readability support
• return the existing error marker when simplification still produces empty content

Fixes #4199.

Tests

• cd src/fetch && uv run pytest tests/test_server.py -q
• cd src/fetch && uv run ruff check src/mcp_server_fetch/server.py src/mcp_server_fetch/init.py tests/test_server.py
• git diff --check