[Bug]: Drag element, event onDragStart

### Describe the bug / 问题描述

Incorrect processing of the FrontElement graph function, when capturing a combo, it does not take into account the fact that the nodes lying in it may contain edges from the outside


From the function getSubgraphRelatedEdges only the array of internal is taken, not taking into account the external, because of which visually edges that stretch from other nodes to combo nodes visually go to the buildings of the background

### Reproduction link / 复现链接

https://stackblitz.com/edit/vitejs-vite-c142bq89?file=src%2FApp.vue

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

Trigger the onDragStart event on the combo and you will see how edge nodes stretching to nodes inside the combo get a smaller zIndex and fade into the background

### G6 Version / G6 版本

🆕 5.x

### OS / 操作系统

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

### Browser / 浏览器

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

Start with the linked StackBlitz reproduction and trace the onDragStart path through FrontElement and getSubgraphRelatedEdges. Confirm the external edges connected to nodes inside the combo retain the correct zIndex and remain visible when the combo is dragged.

Written by the indexing model from the issue text.