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/account_test_service.go

@@ -1564,7 +1564,7 @@ func (s *AccountTestService) sendErrorAndEnd(c *gin.Context, errorMsg string) er
 
 // RunTestBackground executes an account test in-memory (no real HTTP client),
 // capturing SSE output via httptest.NewRecorder, then parses the result.
-func (s *AccountTestService) RunTestBackground(ctx context.Context, accountID int64, modelID string) (*ScheduledTestOutcome, error) {
+func (s *AccountTestService) RunTestBackground(ctx context.Context, accountID int64, modelID string) (*ScheduledTestResult, error) {
 	startedAt := time.Now()
 
 	w := httptest.NewRecorder()
@@ -1574,28 +1574,25 @@ func (s *AccountTestService) RunTestBackground(ctx context.Context, accountID in
 	testErr := s.TestAccountConnection(ginCtx, accountID, modelID)
 
 	finishedAt := time.Now()
-	latencyMs := finishedAt.Sub(startedAt).Milliseconds()
-
 	body := w.Body.String()
 	responseText, errMsg := parseTestSSEOutput(body)
 
-	outcome := &ScheduledTestOutcome{
-		Status:       "success",
-		ResponseText: responseText,
-		ErrorMessage: errMsg,
-		LatencyMs:    latencyMs,
-		StartedAt:    startedAt,
-		FinishedAt:   finishedAt,
-	}
-
+	status := "success"
 	if testErr != nil || errMsg != "" {
-		outcome.Status = "failed"
+		status = "failed"
 		if errMsg == "" && testErr != nil {
-			outcome.ErrorMessage = testErr.Error()
+			errMsg = testErr.Error()
 		}
 	}
 
-	return outcome, nil
+	return &ScheduledTestResult{
+		Status:       status,
+		ResponseText: responseText,
+		ErrorMessage: errMsg,
+		LatencyMs:    finishedAt.Sub(startedAt).Milliseconds(),
+		StartedAt:    startedAt,
+		FinishedAt:   finishedAt,
+	}, nil
 }
 
 // parseTestSSEOutput extracts response text and error message from captured SSE output.