test(rest): pin /v1/actions/snapshot/multi 501 contract

Cozystack does not run multi-RD atomic snapshots; the endpoint is
intentionally 501 Not Implemented so golinstor can distinguish it
from an unknown-route bug. Test locks the contract in.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps

pkg/rest/snapshot_multi_test.go

@@ -0,0 +1,46 @@
+/*
+Copyright 2026 Cozystack contributors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package rest
+
+import (
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/cozystack/blockstor/pkg/store"
+)
+
+// TestSnapshotMultiReturns501 pins the multi-RD snapshot endpoint as
+// explicitly out-of-scope. golinstor expects a deterministic
+// non-success here; falling through to 404 (the previous behaviour)
+// is harder to distinguish from an unknown-route bug.
+func TestSnapshotMultiReturns501(t *testing.T) {
+	base, stop := startServerWithStore(t, store.NewInMemory())
+	defer stop()
+
+	resp := httpPost(t, base+"/v1/actions/snapshot/multi",
+		[]byte(`{"resource_definitions":["pvc-1"],"snapshot_name":"snap-1"}`))
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusNotImplemented {
+		t.Errorf("status: got %d, want 501", resp.StatusCode)
+	}
+
+	if got := resp.Header.Get("Content-Type"); !strings.HasPrefix(got, "application/json") {
+		t.Errorf("Content-Type: got %q, want application/json prefix", got)
+	}
+}