Nodes with image preview missing preview when put in subgraph

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [x] I have custom nodes enabled

### What happened?

Nodes (standard or custom) such as Load Image, SEGSPreview, etc are missing the preview canvas when you make them part of a subgraph. If you edit subgraph widgets a couple of times it eventuallys shows and you can enable it.

### Steps to Reproduce

1. Add Load Image node
2. Add Resize Image
3. Make a subgraph out of those nodes

### How is this affecting you?

Feature doesn't work as expected

### ComfyUI Frontend Version

1.43.18

### Browser

Chrome/Chromium

### Console Errors

```javascript

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### Logs

```shell

```

### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-12198-Nodes-with-image-preview-missing-preview-when-put-in-subgraph-35e6d73d36508153bc84d4e7516e8872) by [Unito](https://www.unito.io)

Reproduce the issue in Chrome/Chromium with Load Image and Resize Image nodes, then place them in a subgraph and inspect the subgraph widget-editing and image-preview entry points. Compare the initial render with the state after editing widgets; done means the preview canvas appears and remains available without repeated edits.

Written by the indexing model from the issue text.