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

[LuuOW]

Technical audit: Verified MCP server implementation for consistency with current SDK patterns.

[he-yufeng]

Superseded by #4281, which fixes the same "Node missing → Readability.js unavailable" problem (#4199) with an automatic Python fallback rather than a new opt-out flag. Consolidating on that one so there aren't three PRs touching the same fetch files — closing this.