proxmox: config block, IDs, marks, state mapping, classify, registration

Phase 2 of the proxmox provider plan. Adds the proxmox: config block with
its validation, registers the type in the config allow-list and the
constructor map, and starts internal/provider/proxmox: the api seam and
its data types, providerIDs (vms/<vmid>/<name>) with the UID-derived VMID,
the description mark, state mapping, and error classification.

Tests read the responses captured from pv1-1 in Phase 0 rather than
repeating Proxmox's wording, and every matcher is run against every
captured error so none fires on the wrong message. The contract methods
are Transient stubs until Phases 5 to 7.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/provider/config.go

@@ -25,6 +25,8 @@ type ProviderConfig struct {
 	// KVM2 is the same block shape as KVM, for the kvm2 provider type: one
 	// or more libvirt/KVM hypervisors reached over SSH.
 	KVM2 *KVMConfig `json:"kvm2,omitempty"`
+	// Proxmox is a Proxmox VE cluster reached through its REST API.
+	Proxmox *ProxmoxConfig `json:"proxmox,omitempty"`
 }
 
 // setBlocks names the type-specific blocks that are non-nil, so validate can
@@ -43,6 +45,9 @@ func (p ProviderConfig) setBlocks() []string {
 	if p.KVM2 != nil {
 		out = append(out, "kvm2")
 	}
+	if p.Proxmox != nil {
+		out = append(out, "proxmox")
+	}
 	return out
 }
 
@@ -106,7 +111,7 @@ func (c *Config) validate() error {
 
 		prefix := fmt.Sprintf("providers[%d] %q", i, p.Name)
 		switch p.Type {
-		case "kubernetes", "gcp", "kvm", "kvm2":
+		case "kubernetes", "gcp", "kvm", "kvm2", "proxmox":
 		case "":
 			return fmt.Errorf("%s: type is required", prefix)
 		default:
@@ -130,6 +135,10 @@ func (c *Config) validate() error {
 			if err := validateKVMBlock(prefix, "kvm2", p.KVM2); err != nil {
 				return err
 			}
+		case "proxmox":
+			if err := validateProxmox(prefix, p.Proxmox); err != nil {
+				return err
+			}
 		}
 	}
 	return nil