Drag-and-drop image into Load Image node does not work inside subgraph

### Custom Node Testing

- [x] I have tried disabling custom nodes and the issue persists (see [how to disable custom nodes](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled) if you need help)

### Expected Behavior

Load Image nodes should function the same whether inside or outside of subgraphs.

### Actual Behavior

When placing a Load Image node inside of a subgraph, drag-and-drop of an image does not work.

### Steps to Reproduce

In a new workflow, create two Load Image nodes. Place one inside of a subgraph. Try to drag-and-drop an image into each Load Image node. The one outside of the subgraph works as expected, but the one inside the subgraph fails; it just opens the dropped image workflow.

### Debug Logs

```powershell
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```

### Other

_No response_

Start by reproducing the issue in a new workflow with two Load Image nodes, placing one inside a subgraph and comparing both drop behaviors. Trace the drag-and-drop handling for the Load Image node and subgraph context; done means an image dropped on either node loads the image instead of opening its workflow.

Written by the indexing model from the issue text.