The health monitor's _basic_site_health_check() used split("_", 1) to parse
project IDs like "wordpress_advanced_site1", incorrectly yielding
plugin_type="wordpress" and site_id="advanced_site1". Now uses SiteManager's
list_all_sites() for proper full_id lookup, and creates a temporary plugin
instance for proper health checks (REST API, WP-CLI) instead of basic HTTP.
Also fixes the same split bug in SiteManager.get_effective_path_suffix().
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>