fix(qa): ISSUE-006 — 新增字段编码唯一性校验，避免创建重复字段

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

common/report/api.go

@@ -362,6 +362,20 @@ func (s *ReportService) SaveField(ctx context.Context, req *model.SaveFieldReq)
 		field.FilterOperators = []string{"=", "!=", ">", "<", ">=", "<=", "IN", "LIKE", "BETWEEN"}
 	}
 
+	// 新增时校验字段编码唯一性
+	if req.ID == nil || *req.ID == 0 {
+		existing, err := s.configLoader.GetReportFields(ctx, req.BusinessCode, req.ReportCode)
+		if err != nil {
+			return nil, fmt.Errorf("校验字段编码失败: %w", err)
+		}
+		allFields := append(append(existing.Dimensions, existing.Indicators...), existing.Filters...)
+		for _, f := range allFields {
+			if f.FieldCode == req.FieldCode && f.Status == model.StatusActive {
+				return nil, fmt.Errorf("字段编码 %s 已存在（业务: %s, 报表: %s）", req.FieldCode, req.BusinessCode, req.ReportCode)
+			}
+		}
+	}
+
 	if req.ID != nil && *req.ID > 0 {
 		field.Id = *req.ID
 		if err := s.configLoader.UpdateField(ctx, field); err != nil {