[Bug] WHITE_WEBDAV_ENDPOINTS环境变量设置上了还是无法进行webdav同步

### 📦 Deployment Method

Vercel

### 📌 Version

v2.15.7
v2.15.8

### 💻 Operating System

Windows

### 📌 System Version

win11

### 🌐 Browser

Chrome

### 📌 Browser Version

130.0.6723.70

### 🐛 Bug Description

WHITE_WEBDAV_ENDPOINTS=https://dav.jianguoyun.com/dav
设置之后也进行了重新部署
webdav地址=https://dav.jianguoyun.com/dav/
用户名设置了
密码也设置了
检查可用性 报错error code 520

### 📷 Recurrence Steps

_No response_

### 🚦 Expected Behavior

_No response_

### 📝 Additional Information

_No response_

The issue names no source file, test, or entry point. Start by reproducing the Vercel deployment using WHITE_WEBDAV_ENDPOINTS with the reported WebDAV credentials and inspect where the availability check returns error 520. Done means the configured endpoint can be checked successfully and WebDAV synchronization works on the reported versions and environment.

Written by the indexing model from the issue text.