Fix flaky test_contentnode_tree on PostgreSQL

Match API response nodes to expected nodes by id instead of relying on
positional ordering, since fixture data has duplicate lft values for
sibling nodes causing nondeterministic ORDER BY results on PostgreSQL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rtibblesbot

kolibri/core/content/test/test_content_app.py

@@ -491,13 +491,17 @@ def test_contentnode_list_long(self):
 
     def _recurse_and_assert(self, data, nodes, recursion_depth=0):
         recursion_depths = []
-        for actual, expected in zip(data, nodes):
+        nodes_by_id = {n.id: n for n in nodes}
+        for actual in data:
+            expected = nodes_by_id[actual["id"]]
             children = actual.pop("children", None)
             self._assert_node(actual, expected)
             if children:
-                child_nodes = content.ContentNode.objects.filter(
-                    available=True, parent=expected
-                ).order_by("lft")
+                child_nodes = list(
+                    content.ContentNode.objects.filter(
+                        available=True, parent=expected
+                    ).order_by("lft")
+                )
                 if children["more"] is None:
                     self.assertEqual(len(child_nodes), len(children["results"]))
                 else: