fix: add groupExistenceBatchReader interface and update test stub for release branch compatibility

backend/internal/service/admin_service.go

@@ -336,6 +336,10 @@ type ProxyExitInfoProber interface {
 	ProbeProxy(ctx context.Context, proxyURL string) (*ProxyExitInfo, int64, error)
 }
 
+type groupExistenceBatchReader interface {
+	ExistsByIDs(ctx context.Context, ids []int64) (map[int64]bool, error)
+}
+
 type proxyQualityTarget struct {
 	Target          string
 	URL             string