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])