[Bug]: no drag-edge behavior

### Describe the bug / 问题描述

I wanna implement the drag-canvas behavior when the user actually drags an edge. I cannot achieve it cause there is no drag edge behavior and no drag_start and drag_end. How can I achieve it now? Help me pls

### Reproduction link / 复现链接

_No response_

### Steps to Reproduce the Bug or Issue / 重现步骤

_No response_

### Version / 版本

Please select / 请选择

### OS / 操作系统

- [ ] macOS
- [ ] Windows
- [ ] Linux
- [ ] Others / 其他

### Browser / 浏览器

- [ ] Chrome
- [ ] Edge
- [ ] Firefox
- [ ] Safari (Limited support / 有限支持)
- [ ] IE (Nonsupport / 不支持)
- [ ] Others / 其他

Review G6's existing drag-canvas and edge-interaction behavior to determine whether dragging an edge is supported and how drag_start and drag_end are exposed. Reproduce the reported interaction in a minimal graph, then define the expected edge-drag behavior and event lifecycle before identifying the relevant implementation and tests.

Written by the indexing model from the issue text.