## 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
Introduce OAuthRefreshAPI as the single entry point for all OAuth token
refresh operations, eliminating the race condition where background
refresh and inline refresh could simultaneously use the same
refresh_token (fixes#1035).
Key changes:
- Add OAuthRefreshExecutor interface extending TokenRefresher with CacheKey
- Add OAuthRefreshAPI.RefreshIfNeeded with lock → DB re-read → double-check flow
- Add ProviderRefreshPolicy / BackgroundRefreshPolicy strategy types
- Simplify all 4 TokenProviders to delegate to OAuthRefreshAPI
- Rewrite TokenRefreshService.refreshWithRetry to use unified API path
- Add MergeCredentials and BuildClaudeAccountCredentials helpers
- Add 40 unit tests covering all new and modified code paths
- 新增并接通 Sora 专用 OAuth 接口与 ST/RT 换取能力
- 完成前端 Sora 授权、RT/ST 手动导入与账号创建流程
- 强化 Sora token 恢复、转发日志与网关路由隔离行为
- 补充后端服务层与路由层相关测试覆盖
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
