fix: 修复考试题目选项数据缺失问题

- 在GetExam API中使用convertToDTO函数转换题目数据
- 确保选项数据(options)正确解析并返回给前端
- 修复前端ExamOnline渲染选择题时的undefined错误

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/handlers/exam_handler.go

@@ -140,6 +140,9 @@ func GetExam(c *gin.Context) {
 		return
 	}
 
+	// 是否显示答案
+	showAnswer := c.Query("show_answer") == "true"
+
 	// 查询题目详情
 	var questions []models.PracticeQuestion
 	if err := db.Where("id IN ?", questionIDs).Find(&questions).Error; err != nil {
@@ -148,24 +151,20 @@ func GetExam(c *gin.Context) {
 		return
 	}
 
-	// 按原始顺序排序题目
+	// 按原始顺序排序题目并转换为DTO
 	questionMap := make(map[uint]models.PracticeQuestion)
 	for _, q := range questions {
 		questionMap[q.ID] = q
 	}
-	orderedQuestions := make([]models.PracticeQuestion, 0, len(questionIDs))
+	orderedDTOs := make([]models.PracticeQuestionDTO, 0, len(questionIDs))
 	for _, id := range questionIDs {
 		if q, ok := questionMap[id]; ok {
-			orderedQuestions = append(orderedQuestions, q)
+			dto := convertToDTO(q)
-		}
-	}
-
 			// 是否显示答案
-	showAnswer := c.Query("show_answer") == "true"
 			if !showAnswer {
-		// 不显示答案时，隐藏答案字段
+				dto.Answer = nil
-		for i := range orderedQuestions {
+			}
-			orderedQuestions[i].AnswerData = ""
+			orderedDTOs = append(orderedDTOs, dto)
 		}
 	}
 
@@ -173,7 +172,7 @@ func GetExam(c *gin.Context) {
 		"success": true,
 		"data": gin.H{
 			"exam":      exam,
-			"questions": orderedQuestions,
+			"questions": orderedDTOs,
 		},
 	})
 }