[Bug]: MMB-click on slot still creates an old reroute

### 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)

### Frontend Version

1.21.7

### Expected Behavior

When middle-clicking on an output slot, a new dot-like reroute should appear.

### Actual Behavior

Instead, this action still creates an old-style reroute, so it's hard to create a new dot connected to an existing output, when an area is crowded with connections.

### Steps to Reproduce

- MMB-click on any output slot of any node
- old node-like reroute appears

### Debug Logs

```powershell
python -s /BigData/AI/ComfyUI/main.py --listen --disable-all-custom-nodes Checkpoint files will always be loaded safely.
Total VRAM 24078 MB, total RAM 31869 MB
pytorch version: 2.7.1+cu128
Set vram state to: NORMAL_VRAM
Device: cuda:0 NVIDIA GeForce RTX 4090 : cudaMallocAsync
Using pytorch attention
Python version: 3.12.11 (main, Jun 6 2025, 06:38:58) [GCC 15.1.1 20250425]
ComfyUI version: 0.3.41
ComfyUI frontend version: 1.21.7
[Prompt Server] web root: /BigData/AI/ComfyUI/venv/lib/python3.12/site-packages/comfyui_frontend_package/static
Skipping loading of custom nodes
Context impl SQLiteImpl.
Will assume non-transactional DDL.
No target revision found.
Starting server

To see the GUI go to: http://0.0.0.0:8188
To see the GUI go to: http://[::]:8188
```

### Browser Logs

[console-export-2025-6-23_21-46-37.txt](https://github.com/user-attachments/files/20869192/console-export-2025-6-23_21-46-37.txt)

### Setting JSON

[comfy.settings.json](https://github.com/user-attachments/files/20869208/comfy.settings.json)

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

Mozilla Firefox

### Other Information

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-4255-Bug-MMB-click-on-slot-still-creates-an-old-reroute-21b6d73d365081e28706f55da8c3558d) by [Unito](https://www.unito.io)

Start by tracing the frontend handler for middle-clicking an output slot, reproducing the issue with custom nodes disabled in Firefox. Compare the old node-like reroute created by this action with the expected dot-like reroute behavior. Done means MMB-clicking any output slot creates the new dot-like reroute, including when the area has existing connections.

Written by the indexing model from the issue text.