refactor: 移除冗余中间类型和不必要代码

- 移除 ScheduledTestOutcome 中间类型，RunTestBackground 直接返回 *ScheduledTestResult
- 简化 SaveResult 直接接受 *ScheduledTestResult
- 移除 handler 中不必要的 nil 检查
- 移除前端 ScheduledTestsPanel 中多余的 String() 转换

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/scheduled_test_port.go

@@ -32,16 +32,6 @@ type ScheduledTestResult struct {
 	CreatedAt    time.Time `json:"created_at"`
 }
 
-// ScheduledTestOutcome is returned by RunTestBackground.
-type ScheduledTestOutcome struct {
-	Status       string
-	ResponseText string
-	ErrorMessage string
-	LatencyMs    int64
-	StartedAt    time.Time
-	FinishedAt   time.Time
-}
-
 // ScheduledTestPlanRepository defines the data access interface for test plans.
 type ScheduledTestPlanRepository interface {
 	Create(ctx context.Context, plan *ScheduledTestPlan) (*ScheduledTestPlan, error)