fix: preserve scalar items in list flatten

Fixes #5705

app/utils/structures.py

@@ -50,7 +50,11 @@ class ListUtils:
         if not any(isinstance(sublist, list) for sublist in nested_list):
             return nested_list
 
-        return [item for sublist in nested_list if isinstance(sublist, list) for item in sublist]
+        return [
+            item
+            for sublist in nested_list
+            for item in (sublist if isinstance(sublist, list) else [sublist])
+        ]
 
 
 class SetUtils:

tests/test_structures.py

@@ -0,0 +1,15 @@
+from unittest import TestCase
+
+from app.utils.structures import ListUtils
+
+
+class ListUtilsTest(TestCase):
+    def test_flatten_keeps_scalar_items_in_mixed_list(self):
+        self.assertEqual(ListUtils.flatten([1, [2, 3], 4]), [1, 2, 3, 4])
+
+    def test_flatten_returns_plain_list_unchanged(self):
+        source = [1, 2, 3]
+        self.assertEqual(ListUtils.flatten(source), source)
+
+    def test_flatten_rejects_non_list_input(self):
+        self.assertEqual(ListUtils.flatten("1,2,3"), [])