fix(template-folder): bound cycle-walk to defend against malformed graphs

src/ScadaLink.TemplateEngine/Services/TemplateFolderService.cs

@@ -86,13 +86,17 @@ public class TemplateFolderService
                 return Result<TemplateFolder>.Failure($"Target folder with ID {newParentId.Value} not found.");
 
             var all = await _repository.GetAllFoldersAsync(cancellationToken);
-            // Walk up from newParentId — if we encounter folderId, the move would create a cycle.
             var byId = all.ToDictionary(f => f.Id);
             var cursor = newParentId;
+            // Walk up from newParentId — if we encounter folderId, the move would create a cycle.
+            // Bound iterations by byId.Count to defensively terminate on a malformed graph.
+            var iterations = 0;
             while (cursor.HasValue)
             {
                 if (cursor.Value == folderId)
                     return Result<TemplateFolder>.Failure("Cannot move a folder under one of its descendants (cycle).");
+                if (++iterations > byId.Count)
+                    return Result<TemplateFolder>.Failure("Folder hierarchy contains a cycle; cannot determine ancestry.");
                 cursor = byId.TryGetValue(cursor.Value, out var node) ? node.ParentFolderId : null;
             }