test: 添加单账号 503 退避重试机制的单元测试

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>

backend/internal/handler/gateway_handler_single_account_retry_test.go

+package handler
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// ---------------------------------------------------------------------------
+// sleepAntigravitySingleAccountBackoff 测试
+// ---------------------------------------------------------------------------
+
+func TestSleepAntigravitySingleAccountBackoff_ReturnsTrue(t *testing.T) {
+	ctx := context.Background()
+	start := time.Now()
+	ok := sleepAntigravitySingleAccountBackoff(ctx, 1)
+	elapsed := time.Since(start)
+
+	require.True(t, ok, "should return true when context is not canceled")
+	// 固定延迟 2s
+	require.GreaterOrEqual(t, elapsed, 1500*time.Millisecond, "should wait approximately 2s")
+	require.Less(t, elapsed, 5*time.Second, "should not wait too long")
+}
+
+func TestSleepAntigravitySingleAccountBackoff_ContextCanceled(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // 立即取消
+
+	start := time.Now()
+	ok := sleepAntigravitySingleAccountBackoff(ctx, 1)
+	elapsed := time.Since(start)
+
+	require.False(t, ok, "should return false when context is canceled")
+	require.Less(t, elapsed, 500*time.Millisecond, "should return immediately on cancel")
+}
+
+func TestSleepAntigravitySingleAccountBackoff_FixedDelay(t *testing.T) {
+	// 验证不同 retryCount 都使用固定 2s 延迟
+	ctx := context.Background()
+
+	start := time.Now()
+	ok := sleepAntigravitySingleAccountBackoff(ctx, 5)
+	elapsed := time.Since(start)
+
+	require.True(t, ok)
+	// 即使 retryCount=5，延迟仍然是固定的 2s
+	require.GreaterOrEqual(t, elapsed, 1500*time.Millisecond)
+	require.Less(t, elapsed, 5*time.Second)
+}