Fix: wrong user count for compositing node tree

**How to reproduce:**
1. Assign a node tree to the scene compositing node group in Python
2. Notice how the node tree has the wrong user count

Example:
```python
C.scene.compositing_node_group = \
    D.node_groups.new("ntree", "CompositorNodeTree")
print(C.scene.compositing_node_group.users) # returns 0
```

Pull Request: https://projects.blender.org/blender/blender/pulls/143577

tests/python/CMakeLists.txt

@@ -1035,6 +1035,11 @@ if(WITH_GPU_COMPOSITOR_TESTS AND TEST_SRC_DIR_EXISTS)
   endif()
 endif()
 
+add_blender_test(
+  compositing_node_group
+  --python ${CMAKE_CURRENT_LIST_DIR}/compositing_node_group.py
+)
+
 # ------------------------------------------------------------------------------
 # GEOMETRY NODE TESTS
 # ------------------------------------------------------------------------------

tests/python/compositing_node_group.py

@@ -0,0 +1,26 @@
+# SPDX-FileCopyrightText: 2025 Blender Authors
+#
+# SPDX-License-Identifier: Apache-2.0
+
+import bpy
+import unittest
+
+
+class CompositingNodeGroupTest(unittest.TestCase):
+    """
+    Tests specific to the root compositing node tree
+    """
+
+    def setUp(self):
+        bpy.ops.wm.read_factory_settings(use_empty=True)
+
+    def test_assign_invalid(self):
+        scene = bpy.data.scenes["Scene"]
+        with self.assertRaises(RuntimeError):
+            scene.compositing_node_group = bpy.data.node_groups.new("invalid", "GeometryNodeTree")
+
+
+if __name__ == "__main__":
+    import sys
+    sys.argv = [__file__] + (sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else [])
+    unittest.main()