refactor: extract failover error handling into shared HandleFailoverError

- Extract duplicated failover error handling from gateway_handler.go (Gemini-compat & Claude paths) and gemini_v1beta_handler.go into shared failover_loop.go
- Introduce TempUnscheduler interface for testability (GatewayService implicitly satisfies it)
- Add comprehensive unit tests for HandleFailoverError (32 test cases covering all paths)
- Fix golangci-lint issues: errcheck in test type assertion, staticcheck QF1003 if/else→switch

backend/internal/handler/failover_loop.go

@@ -0,0 +1,125 @@
+package handler
+
+import (
+	"context"
+	"log"
+	"time"
+
+	"sub2api/internal/service"
+)
+
+// TempUnscheduler 用于 HandleFailoverError 中同账号重试耗尽后的临时封禁。
+// GatewayService 隐式实现此接口。
+type TempUnscheduler interface {
+	TempUnscheduleRetryableError(ctx context.Context, accountID int64, failoverErr *service.UpstreamFailoverError)
+}
+
+// FailoverAction 表示 failover 错误处理后的下一步动作
+type FailoverAction int
+
+const (
+	// FailoverRetry 同账号重试（调用方应 continue 重新进入循环，不更换账号）
+	FailoverRetry FailoverAction = iota
+	// FailoverSwitch 切换账号（调用方应 continue 重新选择账号）
+	FailoverSwitch
+	// FailoverExhausted 切换次数耗尽（调用方应返回错误响应）
+	FailoverExhausted
+	// FailoverCanceled context 已取消（调用方应直接 return）
+	FailoverCanceled
+)
+
+const (
+	// maxSameAccountRetries 同账号重试次数上限（针对 RetryableOnSameAccount 错误）
+	maxSameAccountRetries = 2
+	// sameAccountRetryDelay 同账号重试间隔
+	sameAccountRetryDelay = 500 * time.Millisecond
+)
+
+// FailoverState 跨循环迭代共享的 failover 状态
+type FailoverState struct {
+	SwitchCount           int
+	MaxSwitches           int
+	FailedAccountIDs      map[int64]struct{}
+	SameAccountRetryCount map[int64]int
+	LastFailoverErr       *service.UpstreamFailoverError
+	ForceCacheBilling     bool
+	hasBoundSession       bool
+}
+
+// NewFailoverState 创建 failover 状态
+func NewFailoverState(maxSwitches int, hasBoundSession bool) *FailoverState {
+	return &FailoverState{
+		MaxSwitches:           maxSwitches,
+		FailedAccountIDs:      make(map[int64]struct{}),
+		SameAccountRetryCount: make(map[int64]int),
+		hasBoundSession:       hasBoundSession,
+	}
+}
+
+// HandleFailoverError 处理 UpstreamFailoverError，返回下一步动作。
+// 包含：缓存计费判断、同账号重试、临时封禁、切换计数、Antigravity 延时。
+func (s *FailoverState) HandleFailoverError(
+	ctx context.Context,
+	gatewayService TempUnscheduler,
+	accountID int64,
+	platform string,
+	failoverErr *service.UpstreamFailoverError,
+) FailoverAction {
+	s.LastFailoverErr = failoverErr
+
+	// 缓存计费判断
+	if needForceCacheBilling(s.hasBoundSession, failoverErr) {
+		s.ForceCacheBilling = true
+	}
+
+	// 同账号重试：对 RetryableOnSameAccount 的临时性错误，先在同一账号上重试
+	if failoverErr.RetryableOnSameAccount && s.SameAccountRetryCount[accountID] < maxSameAccountRetries {
+		s.SameAccountRetryCount[accountID]++
+		log.Printf("Account %d: retryable error %d, same-account retry %d/%d",
+			accountID, failoverErr.StatusCode, s.SameAccountRetryCount[accountID], maxSameAccountRetries)
+		if !sleepWithContext(ctx, sameAccountRetryDelay) {
+			return FailoverCanceled
+		}
+		return FailoverRetry
+	}
+
+	// 同账号重试用尽，执行临时封禁
+	if failoverErr.RetryableOnSameAccount {
+		gatewayService.TempUnscheduleRetryableError(ctx, accountID, failoverErr)
+	}
+
+	// 加入失败列表
+	s.FailedAccountIDs[accountID] = struct{}{}
+
+	// 检查是否耗尽
+	if s.SwitchCount >= s.MaxSwitches {
+		return FailoverExhausted
+	}
+
+	// 递增切换计数
+	s.SwitchCount++
+	log.Printf("Account %d: upstream error %d, switching account %d/%d",
+		accountID, failoverErr.StatusCode, s.SwitchCount, s.MaxSwitches)
+
+	// Antigravity 平台换号线性递增延时
+	if platform == service.PlatformAntigravity {
+		if !sleepFailoverDelay(ctx, s.SwitchCount) {
+			return FailoverCanceled
+		}
+	}
+
+	return FailoverSwitch
+}
+
+// sleepWithContext 等待指定时长，返回 false 表示 context 已取消。
+func sleepWithContext(ctx context.Context, d time.Duration) bool {
+	if d <= 0 {
+		return true
+	}
+	select {
+	case <-ctx.Done():
+		return false
+	case <-time.After(d):
+		return true
+	}
+}