fix(教师表单): 修复章节选择验证后未清空选中章节的问题

在验证章节时间有效性失败时，清空已选章节以避免用户困惑

@@ -2,7 +2,7 @@
  * @Author: hookehuyr hookehuyr@gmail.com
  * @Date: 2025-01-20 10:00:00
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-06-25 15:49:50
+ * @LastEditTime: 2025-06-25 16:27:12
  * @FilePath: /mlaj/src/views/teacher/formPage.vue
  * @Description: 教师作业新增表单页面
 -->
@@ -675,6 +675,7 @@ const confirmChapterSelection = () => {
   const invalidChapters = selectedChapters.value.filter(chapter => !chapter.startTime || !chapter.endTime);
   if (invalidChapters.length > 0) {
     showToast('请为所有选中的章节设置开始和结束时间');
+    selectedChapters.value = '';
     return;
   }
 
@@ -684,6 +685,7 @@ const confirmChapterSelection = () => {
   );
   if (timeInvalidChapters.length > 0) {
     showToast('存在结束时间早于开始时间的章节，请重新设置');
+    selectedChapters.value = '';
     return;
   }