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## Problem

When a proxy is unreachable, token refresh retries up to 4 times with
30s timeout each, causing requests to hang for ~2 minutes before
failing with a generic 502 error. The failed account is not marked,
so subsequent requests keep hitting it.

## Changes

### Proxy connection fast-fail
- Set TCP dial timeout to 5s and TLS handshake timeout to 5s on
  antigravity client, so proxy connectivity issues fail within 5s
  instead of 30s
- Reduce overall HTTP client timeout from 30s to 10s
- Export `IsConnectionError` for service-layer use
- Detect proxy connection errors in `RefreshToken` and return
  immediately with "proxy unavailable" error (no retries)

### Token refresh temp-unschedulable
- Add 8s context timeout for token refresh on request path
- Mark account as temp-unschedulable for 10min when refresh fails
  (both background `TokenRefreshService` and request-path
  `GetAccessToken`)
- Sync temp-unschedulable state to Redis cache for immediate
  scheduler effect
- Inject `TempUnschedCache` into `AntigravityTokenProvider`

### Account failover
- Return `UpstreamFailoverError` on `GetAccessToken` failure in
  `Forward`/`ForwardGemini` to trigger handler-level account switch
  instead of returning 502 directly

### Proxy probe alignment
- Apply same 5s dial/TLS timeout to shared `httpclient` pool
- Reduce proxy probe timeout from 30s to 10s

提取 proxyurl.Parse() 公共包，将分散在 6 处的代理 URL 验证逻辑
统一收敛，确保无效代理配置在创建时立即失败，永不静默回退直连。

主要变更：
- 新增 proxyurl 包：统一 TrimSpace → url.Parse → Host 校验 → Scheme 白名单
- socks5:// 自动升级为 socks5h://，防止 DNS 泄漏（大小写不敏感）
- antigravity: http.ProxyURL → proxyutil.ConfigureTransportProxy 支持 SOCKS5
- openai_oauth: 删除 newOpenAIOAuthHTTPClient，收编至 httpclient.GetClient
- 移除未使用的 ProxyStrict 字段（fail-fast 已是全局默认行为）
- 补充 15 个 proxyurl 测试 + pricing/usage fail-fast 测试

- 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

- 新增 OnboardUser API 客户端方法，支持账号 onboarding 获取 project_id
- loadProjectIDWithRetry 增加 onboard 回退：LoadCodeAssist 未返回 project_id 时自动触发 onboarding
- GetAccessToken 中 project_id 补齐改用轻量 FillProjectID 替代全量 RefreshAccountToken
- 补齐逻辑增加 5 分钟冷却机制，防止频繁重试
- OnboardUser 轮询等待改为 context 感知，支持提前取消
- 提取 mergeCredentials 辅助方法消除重复代码
- 新增 extractProjectIDFromOnboardResponse 和 resolveDefaultTierID 单元测试

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>