fix: 修复exam_service异常类导入错误

将不存在的BusinessException/ErrorCode替换为现有的NotFoundError/ValidationError

backend/app/services/exam_service.py

@@ -10,7 +10,7 @@ from sqlalchemy import select, func, and_, or_, desc
 from app.models.exam import Exam, Question, ExamResult
 from app.models.exam_mistake import ExamMistake
 from app.models.course import Course, KnowledgePoint
-from app.core.exceptions import BusinessException, ErrorCode
+from app.core.exceptions import NotFoundError, ValidationError
 from app.utils.score_distributor import ScoreDistributor
 
 
@@ -36,7 +36,7 @@ class ExamService:
         # 检查课程是否存在
         course = await db.get(Course, course_id)
         if not course:
-            raise BusinessException(error_code=ErrorCode.NOT_FOUND, message="课程不存在")
+            raise NotFoundError("课程不存在")
 
         # 获取该课程的所有可用题目
         stmt = select(Question).where(
@@ -46,9 +46,7 @@ class ExamService:
         all_questions = result.scalars().all()
 
         if not all_questions:
-            raise BusinessException(
-                error_code=ErrorCode.VALIDATION_ERROR, message="该课程暂无题目"
-            )
+            raise ValidationError("该课程暂无题目")
 
         # 随机选择题目
         selected_questions = random.sample(
@@ -120,12 +118,10 @@ class ExamService:
         exam = result.scalar_one_or_none()
 
         if not exam:
-            raise BusinessException(error_code=ErrorCode.NOT_FOUND, message="考试记录不存在")
+            raise NotFoundError("考试记录不存在")
 
         if exam.status != "started":
-            raise BusinessException(
-                error_code=ErrorCode.VALIDATION_ERROR, message="考试已结束或已提交"
-            )
+            raise ValidationError("考试已结束或已提交")
 
         # 检查考试是否超时
         if datetime.now() > exam.start_time + timedelta(
@@ -133,9 +129,7 @@ class ExamService:
         ):
             exam.status = "timeout"
             await db.commit()
-            raise BusinessException(
-                error_code=ErrorCode.VALIDATION_ERROR, message="考试已超时"
-            )
+            raise ValidationError("考试已超时")
 
         # 处理答案
         answers_dict = {ans["question_id"]: ans["answer"] for ans in answers}
@@ -223,7 +217,7 @@ class ExamService:
         exam = result.scalar_one_or_none()
 
         if not exam:
-            raise BusinessException(error_code=ErrorCode.NOT_FOUND, message="考试记录不存在")
+            raise NotFoundError("考试记录不存在")
 
         # 构建返回数据
         exam_data = {