fix: ExamService.start_exam返回ID避免懒加载

修改start_exam返回exam.id而不是整个Exam对象,
彻底避免SQLAlchemy异步会话的懒加载问题

backend/app/api/v1/exam.py

@@ -66,7 +66,7 @@ async def start_exam(
     user_id = current_user.id
     username = current_user.username
     
-    exam = await ExamService.start_exam(
+    exam_id = await ExamService.start_exam(
         db=db,
         user_id=user_id,
         course_id=request.course_id,
@@ -96,7 +96,7 @@ async def start_exam(
         )
     )
 
-    return ResponseModel(code=200, data=StartExamResponse(exam_id=exam.id), message="考试开始")
+    return ResponseModel(code=200, data=StartExamResponse(exam_id=exam_id), message="考试开始")
 
 
 @router.post("/submit", response_model=ResponseModel[SubmitExamResponse])

backend/app/services/exam_service.py

@@ -20,7 +20,7 @@ class ExamService:
     @staticmethod
     async def start_exam(
         db: AsyncSession, user_id: int, course_id: int, question_count: int = 10
-    ) -> Exam:
+    ) -> int:
         """
         开始考试
 
@@ -31,7 +31,7 @@ class ExamService:
             question_count: 题目数量
 
         Returns:
-            Exam: 考试实例
+            int: 考试ID
         """
         # 检查课程是否存在
         course = await db.get(Course, course_id)
@@ -94,7 +94,9 @@ class ExamService:
         await db.commit()
         await db.refresh(exam)
         
-        return exam
+        # 返回exam.id而不是整个对象，避免懒加载问题
+        exam_id = exam.id
+        return exam_id
 
     @staticmethod
     async def submit_exam(