GitLab

TestConnection now reuses antigravityRetryLoop instead of a standalone
HTTP loop, gaining credits overages, smart retry, and 429/503 backoff
for free. AccountSwitchError is caught and surfaced as a friendly
message. Also populates RateLimitedModel in TempUnscheduled switch error.

Test fixes:
- Use RATE_LIMIT_EXCEEDED in 503 short-delay test to avoid 60x1s timeout
- Clamp waitDuration=0 instead of 999s to avoid 15s max-wait timeout
- Enhance mockSmartRetryUpstream with repeatLast and body caching

- gofmt: 修复 error_passthrough_service.go 格式问题
- errcheck: 修复 error_passthrough_runtime_test.go 类型断言未检查
- staticcheck: if-else 改为 switch (gateway_service.go)
- test: 修复两个测试用例错误使用 MODEL_CAPACITY_EXHAUSTED 导致走错路径

覆盖 Service 层和 Handler 层的所有新增逻辑：
- isSingleAccountRetry context 标记检查
- handleSmartRetry 中 503 + SingleAccountRetry 分支
- handleSingleAccountRetryInPlace 原地重试逻辑
- antigravityRetryLoop 预检查跳过限流
- sleepAntigravitySingleAccountBackoff 固定延迟退避
- 端到端集成场景验证
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>