fix: AI生成弹窗传空内容bug + 补全B07测试覆盖

缺陷修复:
- openGenerateModal() knowledgeBaseContent 从 '' 改为从已有题目拼接
- 修复后点击AI生成→生成按钮不再报400

测试补充(B07b/B07c):
- 弹窗提交不报前端错误(UI验证)
- API级生成接口不返回400(API验证)
通过33/33全部通过

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/components/views/QuestionBankDetailView.tsx

@@ -152,7 +152,9 @@ export default function QuestionBankDetailView() {
 
   const openGenerateModal = () => {
     setShowGenerate(true);
-    setGenerateForm({ count: 5, knowledgeBaseContent: '' });
+    // 从已有题目中拼接内容作为出题素材
+    const content = items.map(i => i.questionText).filter(Boolean).join('\n');
+    setGenerateForm({ count: 5, knowledgeBaseContent: content || '暂无题目内容，请先在题库中添加题目' });
   };
 
   const dimensionOptions = template?.dimensions?.map(d => ({ value: d.name || d.label, label: d.label || d.name }))