OIDExternalUserAgentIOS not run in main tread

**Describe the bug**
`presentationAnchorForWebAuthenticationSession`, -[UIView window] must be used from main thread only

**To Reproduce**
Steps to reproduce the behavior:
1. Go to '...'
2. Tap on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment**
- Device: MacBook Pro 2019
- OS: macOS 15
- Browser: embedded iOS app popup browser

**Additional context**

首先定位 OIDExternalUserAgentIOS 和 presentationAnchorForWebAuthenticationSession 的实现，然后跟踪访问 UIView.window 的位置。复现嵌入式 iOS 应用的弹出浏览器流程，并验证该访问在主线程上进行且不会出现报告中的警告。

由索引模型根据 Issue 内容生成。