feat(class): 删除班级时验证是否存在学生

- 新增StudentDOMapper接口方法selectStudentCountByClassId，用于查询班级下学生数量
- 在ClassServiceImpl中注入StudentDOMapper
- 删除班级时先判断班级下是否存在学生，若存在则抛出异常防止删除
- 更新StudentDOMapper.xml，添加对应的SQL查询语句selectStudentCountByClassId

enlish-service/src/main/resources/mapper/StudentDOMapper.xml

@@ -61,4 +61,11 @@
         set is_deleted = 1
         where id = #{id}
     </update>
+
+    <select id="selectStudentCountByClassId" resultType="java.lang.Integer">
+        select count(1)
+        from student
+        where class_id = #{classId}
+            and is_deleted = 0
+    </select>
 </mapper>