fix: 修复 default_group 配置不生效的问题

[lanrenwo]

• 在初始认证请求(type="init")时,如果客户端未选择组,则使用配置的 default_group
• 验证 default_group 是否存在于启用的组列表中
• 如果配置的组不存在或被禁用,记录警告日志并忽略该配置
• 修复 Closes affected/package: default_group值不生效 #381

[codecov]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 15.45%. Comparing base (1963fef) to head (40ad4b6).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
server/handler/link_auth.go	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
- Coverage   15.49%   15.45%   -0.04%     
==========================================
  Files          75       75              
  Lines        6569     6585      +16     
==========================================
  Hits         1018     1018              
- Misses       5416     5432      +16     
  Partials      135      135              

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