Blackboard variables are not duplicated or pasted when using context menu actions or shortcuts

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Import the attached "MySimpleGraph.cs" into the Project
3. Go to Assets > Create > Graph Toolkit Samples > Simple Graph
4. Open the created graph
5. In the Blackboard, click the "+" button to add a new variable and select it
6. Right-click on the variable
7. Select "Duplicate" or "Copy" and then "Paste"
8. Observe the behavior

Actual results: Nothing happens, the variable is neither copied, pasted, nor duplicated

Expected results: The selected variable is successfully duplicated or pasted into the Blackboard list

Reproducible with versions: 6000.4.1f1, 6000.5.0b2, 6000.6.0a2

Tested on (OS): Ubuntu 24.04, Windows 11, macOS 26.3

Notes:

• Neither keyboard shortcuts nor selecting the actions through the context menu seem to work
• Fails consistently across all tested operating systems