[mob][photos] Add assert to ensure copy is enabled for internal users

mobile/lib/services/collections_service.dart

@@ -32,6 +32,7 @@ import "package:photos/models/files_split.dart";
 import "package:photos/models/metadata/collection_magic.dart";
 import 'package:photos/services/app_lifecycle_service.dart';
 import "package:photos/services/favorites_service.dart";
+import "package:photos/services/feature_flag_service.dart";
 import 'package:photos/services/file_magic_service.dart';
 import 'package:photos/services/local_sync_service.dart';
 import 'package:photos/services/remote_sync_service.dart';
@@ -1161,6 +1162,9 @@ class CollectionsService {
       await _addToCollection(dstCollectionID, splitResult.ownedByCurrentUser);
     }
     if (splitResult.ownedByOtherUsers.isNotEmpty) {
+      if (!FeatureFlagService.instance.isInternalUserOrDebugBuild()) {
+        throw ArgumentError('Cannot add files owned by other users');
+      }
       late final List<EnteFile> filesToCopy;
       late final List<EnteFile> filesToAdd;
       (filesToAdd, filesToCopy) = (await _splitFilesToAddAndCopy(