test: cover missing-id and non-list roots; drop unreachable empty-nodes guard

tests/unit/test_tree_nodes.py

@@ -50,6 +50,24 @@ def test_empty_roots_raises_value_error(tmp_path):
         load_tree_nodes(tmp_path, "vX")
 
 
+def test_node_missing_id_raises_value_error(tmp_path):
+    vdir = tmp_path / "videos" / "vX"
+    vdir.mkdir(parents=True)
+    (vdir / "tree.json").write_text(
+        json.dumps({"roots": [{"card": {}, "time_range": [0, 1]}]}), encoding="utf-8"
+    )
+    with pytest.raises(ValueError):
+        load_tree_nodes(tmp_path, "vX")
+
+
+def test_roots_not_list_raises_value_error(tmp_path):
+    vdir = tmp_path / "videos" / "vX"
+    vdir.mkdir(parents=True)
+    (vdir / "tree.json").write_text(json.dumps({"roots": "not-a-list"}), encoding="utf-8")
+    with pytest.raises(ValueError):
+        load_tree_nodes(tmp_path, "vX")
+
+
 def test_load_for_videos_dedups():
     data = load_tree_data_for_videos(_STORE, [_VID, _VID])
     assert set(data.keys()) == {_VID}