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/proxmox/fixtures_test.go

@@ -0,0 +1,57 @@
+package proxmox
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// capture is one response recorded from the real host in Phase 0: the
+// status, the reason phrase after the status code, and the raw body.
+type capture struct {
+	Status int    `json:"status"`
+	Reason string `json:"reason"`
+	Body   string `json:"body"`
+}
+
+// readCapture loads testdata/<name>.json and fails the test when it is
+// missing, so every string a test relies on is backed by a real response.
+func readCapture(t *testing.T, name string) capture {
+	t.Helper()
+	raw, err := os.ReadFile(filepath.Join("testdata", name+".json"))
+	if err != nil {
+		t.Fatalf("reading capture %s: %v", name, err)
+	}
+	var c capture
+	if err := json.Unmarshal(raw, &c); err != nil {
+		t.Fatalf("parsing capture %s: %v", name, err)
+	}
+	return c
+}
+
+// loadAPIError turns an error capture into the apiError the client builds.
+func loadAPIError(t *testing.T, name string) *apiError {
+	t.Helper()
+	c := readCapture(t, name)
+	if c.Status < 400 {
+		t.Fatalf("capture %s has status %d, want an error status", name, c.Status)
+	}
+	return newAPIError(c.Status, c.Reason, []byte(c.Body))
+}
+
+// decodeData decodes the "data" member of a successful capture into T.
+func decodeData[T any](t *testing.T, name string) T {
+	t.Helper()
+	c := readCapture(t, name)
+	if c.Status != 200 {
+		t.Fatalf("capture %s has status %d, want 200", name, c.Status)
+	}
+	var env struct {
+		Data T `json:"data"`
+	}
+	if err := json.Unmarshal([]byte(c.Body), &env); err != nil {
+		t.Fatalf("decoding capture %s: %v", name, err)
+	}
+	return env.Data
+}