When a Proxmox cluster is discovered, Pulse now includes the user-provided
main host URL as a fallback endpoint. This handles scenarios where Proxmox
reports internal IPs that aren't reachable from Pulse's network (e.g.,
monitoring a remote cluster across different networks).
Previously, if all cluster endpoint IPs were unreachable, the connection
would fail with no fallback. Now the ClusterClient will fall back to the
main host URL, allowing Proxmox to route API calls internally.
Related to #1028
43b5fad12c