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PR #682 (release → main 全量同步) 将 Antigravity 和 Gemini CLI 的
OAuth client_secret 硬编码值替换为了 "GOCSPX-your-client-secret" 占位符，
导致未配置环境变量的部署环境中 token 刷新失败。

恢复内容：
- antigravity/oauth.go: 恢复真实 client_secret
- antigravity/oauth_test.go: 恢复测试断言中的真实值
- geminicli/constants.go: 恢复真实 client_secret

Update the default user-agent version from 1.18.4 to 1.19.6
to match the latest official antigravity client.

The ClientSecret constant was left as an empty string after
getClientSecret() was refactored to use defaultClientSecret.
Remove the dead constant and update the test accordingly.

- Fix oauth_test.go and client_test.go to use defaultClientSecret
  variable instead of env var (init() already sets the default)
- Align migration 058 gemini-3-pro-high/low/preview mappings with
  constants.go (map to 3.1 versions)

- oauth.go: GetUserAgent() 缺少闭合大括号导致语法错误
- client_test.go/oauth_test.go: UserAgent 变量已重构为 GetUserAgent()，更新测试引用
- model_rate_limit_test.go: gemini-3-pro-preview 映射目标已更新为 gemini-3.1-pro-high，同步测试

删除因GitHub计费锁定而失败的CI工作流。
为6个核心Go源文件补充单元测试，全部达到50%以上覆盖率：
- response/response.go: 97.6%
- antigravity/oauth.go: 90.1%
- antigravity/client.go: 88.6% (新增27个HTTP客户端测试)
- geminicli/oauth.go: 91.8%
- service/oauth_service.go: 61.2%
- service/gemini_oauth_service.go: 51.9%

新增/增强8个测试文件，共计5600+行测试代码。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>