[Bug][Regression] Proxy widget in subgraph nodes broken with external input

### Prerequisites

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

### What happened?

When a subgraph node has external input, we cannot modify its proxy widgets for controls. This behavior significantly reduce the usability of subgraph node. After investigation, the bug is located at [here](https://github.com/Comfy-Org/ComfyUI_frontend/blob/7131c274f3f64e797b66482187f4d70fa4076d16/src/lib/litegraph/src/subgraph/SubgraphNode.ts#L251), which is part of the PR #9282 . By always returning False, this regression is solved.

While the fix/workaround is simple, the intention behind `_shouldPersistLinkedOnly` is unclear to me. It's better to have more discussion on the expected behavior before moving forward to opening a new PR. Also cc the author of the PR @DrJKL .

### Steps to Reproduce

1. Create two nodes "Primitive Int" and "Empty Latent Image", connect int node to one of the widgets of the latent.
2. Convert latent node to a subgraph. Now exactly one proxy widget is shown.
3. Try to show other input widget, in the sidebar fields disappeared but they don't appear on the subgraph node itself

### How is this affecting you?

Crashes ComfyUI completely

### ComfyUI Frontend Version

1.42.4

### Browser

Firefox

### Console Errors

```javascript

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### Logs

```shell

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### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-9902-Bug-Regression-Proxy-widget-in-subgraph-nodes-broken-with-external-input-3226d73d365081508d37f51239345d09) by [Unito](https://www.unito.io)