Update extension tests to reflect installed extensions
Extensions are now properly loaded via dune-site plugin mechanism,
so tests expecting "No extensions installed" have been updated to
expect "Installed extensions:" instead.
Also enhanced extension_plugins.t to verify extension-styled output:
- Check for rfc-reference class from rfc extension
- Check for rfc-editor.org links
- Check for admonition-warning class from admonition extension
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>