边的背景色能否设置垂直渐变？

### 问题描述

看了文档，边的背景可以做成水平方向渐变，能否实现垂直渐变的效果？效果如图（下）所示：


### 重现链接

无

### 重现步骤

无

### 预期行为

无

### 平台

- 操作系统: [macOS, Windows, Linux, React Native ...]
- 网页浏览器: [Google Chrome, Safari, Firefox]
- X6 版本: [1.28.2 ... ]

### 屏幕截图或视频（可选）

_No response_

### 补充说明（可选）

_No response_

Start by reading the existing documentation for horizontal edge background gradients and locating the related edge-rendering implementation. Determine how a vertical gradient would be represented and verify that the requested effect works in the diagram editor; the issue provides no named files, tests, or reproduction link.

Written by the indexing model from the issue text.