Always display the node's source handle

Previously, we hid the node's source handle in `WorkflowDiagramStepNodeBase`. Now, we want to always display it for most nodes (we want to keep it hidden for the CreateStep node). The background color should change on hover.

<img width="1000" height="826" alt="Image" src="https://github.com/user-attachments/assets/1800f991-087d-491f-b209-ccbcde639ee6" />