Subgraph instance: connected STRING input renders as disconnected floating dot instead of input slot

## Environment

| | |
|---|---|
| ComfyUI version | 0.18.0 |
| Frontend version | 1.41.21 (comfyui-frontend-package) |
| OS | Windows 11 |
| Browser | Version 146.0.7680.81 (Official Build) (64-bit) |

## Description

When a subgraph instance node has a **STRING-type input that is connected**
to an external node, the input slot does not render on the subgraph instance
node. Instead, a disconnected dot appears at an unrelated position on the
canvas, completely detached from the node.

Non-STRING data inputs (LATENT, IMAGE, etc.) on the **same subgraph instance**
render correctly as connectable input slots.

## Steps to Reproduce

1. Create a subgraph with at least one STRING input and one data input (e.g. LATENT)
2. Place an instance of that subgraph on the canvas
3. Connect an external String node to the STRING input of the subgraph instance
4. **Observe:** the STRING input slot disappears from the node; a small
floating dot appears elsewhere on the canvas, disconnected from the node

The attached workflow (`rendering-bug.json`) reproduces this with two steps:
load the file, observe the canvas.

## Expected Behavior

The STRING input renders as a connectable input slot on the left side of the
subgraph instance node — identical to how LATENT and IMAGE inputs behave.

## Actual Behavior

- The STRING input is **not visible** on the subgraph instance node
- A **floating dot** appears at an unrelated canvas position (see screenshot)
- The LATENT input on the same node renders correctly
- The workflow **executes correctly** — the data flow is intact, only the
visual/interactive rendering is broken

## Additional Notes

- Unconnected STRING inputs (`link: null`) render correctly as **inline
widgets** on the subgraph instance — the bug only manifests when a STRING
input has an external connection
- The issue appears isolated to STRING type (possibly other "widget types"
like INT, FLOAT, COMBO when connected — not verified)
- The STRING output from the String (Multiline) node draws a wire that
terminates at the floating dot, not at the subgraph node

## Attachments

- `rendering-bug.json` — minimal workflow reproducing the issue
- screenshot (see below)

[rendering-bug.json](https://github.com/user-attachments/files/26159811/rendering-bug.json)

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-10368-Subgraph-instance-connected-STRING-input-renders-as-disconnected-floating-dot-instea-32a6d73d3650813cb92bf76234953eef) by [Unito](https://www.unito.io)