Implements testConnection (via /api2/json/version) and listResources
(via /api2/json/cluster/resources) using Proxmox's API token auth header,
following the same pattern as the Docker adapter. Verified end-to-end:
graceful failure against an unreachable host, correct event logging, and
exclusion from the resources endpoint when not connected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BbJV5nm8KPVH1oNJYKpnoF
