[Bug]: Shift-drag on subgraph input node slot creates new link instead of moving existing link

### Frontend Version

main

### Expected Behavior

When shift-dragging on a subgraph input node slot that already has a connection, it should move the existing link (disconnect and allow reconnection), consistent with the behavior of shift-dragging on normal node output slots.

### Actual Behavior

Shift-dragging on a subgraph input node slot creates a new link instead of moving the existing connection. This is inconsistent with the standard node behavior where shift-drag moves existing links.

### Steps to Reproduce

1. Create a subgraph with at least one input slot
2. Connect a node to the subgraph input slot
3. Hold Shift and drag from the subgraph input slot
4. Observe that a new link is created instead of moving the existing connection

### Debug Logs

_No response_

### Browser Logs

_No response_

### What browsers do you use to access the UI ?

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4876-Bug-Shift-drag-on-subgraph-input-node-slot-creates-new-link-instead-of-moving-exist-24a6d73d36508174b0b7ece07f59070a) by [Unito](https://www.unito.io)