Backport of v6 commits a87c9950 and 347d7db1.
Part 1 (a87c9950): Wrap the four guest agent c.get() errors with
fmt.Errorf("guest agent ...: %w", err) so isVMSpecificError() correctly
scopes them to the VM rather than the cluster endpoint.
Part 2 (347d7db1): Replace the 20+ pattern blocklist in
executeWithFailover with an allowlist via isEndpointConnectivityError().
Only true TCP/DNS/TLS failures mark an endpoint unhealthy. Any HTTP
response from Proxmox — including 500 — proves the node is reachable
and returns the error without affecting endpoint health.
