fix(fetch): fall back without readability js

[he-yufeng]

Summary:

• avoid the optional Readability.js path when Node.js is not available
• add a --no-readability-js switch for environments where the Node.js simplifier is installed but problematic
• document the fallback behavior and cover both default and opt-out paths in fetch tests

Fixes #4199.

To verify:

• uv run pytest tests -q
• uv run ruff check src tests
• uv run pyright src
• git diff --check