feat: LLM-generated adaptive follow-up questions

- Grader: LLM outputs follow_up_question targeting uncovered keyPoints
- Remove static followupHints usage in grading flow
- maxFollowUps sourced from question.maxFollowUps (hints.length)
- Clean answerKey: remove followupHints field
- Three-language prompt update with examples and bad examples
- Grader spec: add follow_up_question to mock responses

server/src/assessment/graph/nodes/grader.node.ts

@@ -69,7 +69,6 @@ export const graderNode = async (
 
   const isChoice = currentQuestion.questionType === 'MULTIPLE_CHOICE';
   const expectedAnswer = currentQuestion.correctAnswer;
-  const answerKey = (state.questionAnswerKey as any)?.[currentQuestion.id];
 
   if (isChoice && expectedAnswer) {
     const userAnswer = (lastUserMessage.content as string).trim();
@@ -113,13 +112,20 @@ export const graderNode = async (
 1. 0 到 10 的评分。
 2. 建设性的反馈。
 3. 如果回答不完整或不清晰，需要进一步解释，请将 'should_follow_up' 标志设为 true。
+4. follow_up_question：当 should_follow_up 为 true 时必须填写——针对用户尚未覆盖的关键点提问，不得提问已涵盖的内容。false 时填 null。
 
 请以 JSON 格式返回响应：
-{
-  "score": 8,
-  "feedback": "...",
-  "should_follow_up": false
-}`;
+{"score":0到10,"feedback":"评语","should_follow_up":true或false,"follow_up_question":"追问或null"}
+
+示例（需要追问）：
+{"score":6,"feedback":"提到了安全性和性能，未说明依赖关系。","should_follow_up":true,"follow_up_question":"你如何让AI在计划中明确任务依赖关系？"}
+
+示例（不需追问）：
+{"score":8,"feedback":"回答完整。","should_follow_up":false,"follow_up_question":null}
+
+反面示例（禁止这样做）：
+{"should_follow_up":true,"follow_up_question":"除了这些还有什么？"}
+↑ 用户已列出安全性、性能具体内容，不应再泛泛追问"还有什么"。`;
 
   const systemPromptJa = `あなたは専門的な試験官です。
 以下の質問とキーポイントに基づいて、ユーザーの回答を採点してください。
@@ -142,13 +148,20 @@ export const graderNode = async (
 1. 0 から 10 までのスコア。
 2. 建設的なフィードバック。
 3. 回答が不完全または不明確で、さらなる説明が必要な場合は、'should_follow_up' フラグを true に設定してください。
+4. follow_up_question：should_follow_up が true の場合必須——ユーザーがまだカバーしていないキーポイントに焦点を当て、既に回答済みの内容は質問しないこと。false の場合は null。
 
 JSON 形式で回答してください：
-{
-  "score": 8,
-  "feedback": "...",
-  "should_follow_up": false
-}`;
+{"score":0から10,"feedback":"評価","should_follow_up":trueかfalse,"follow_up_question":"追質問かnull"}
+
+例（追質問が必要）：
+{"score":6,"feedback":"安全性と性能に言及したが、依存関係が不明。","should_follow_up":true,"follow_up_question":"AIに計画内のタスク依存関係を明示させる方法は？"}
+
+例（不要）：
+{"score":8,"feedback":"回答は完全。","should_follow_up":false,"follow_up_question":null}
+
+悪い例：
+{"should_follow_up":true,"follow_up_question":"他に何かありますか？"}
+↑ ユーザーが既に具体的内容を挙げているのに「他に何か」と聞くのは不適切。`;
 
   const systemPromptEn = `You are an expert examiner. 
 Grade the user's answer based on the following question and key points.
@@ -171,13 +184,20 @@ Provide:
 1. A score from 0 to 10.
 2. Constructive feedback.
 3. A boolean flag 'should_follow_up' if the answer is incomplete or unclear and needs further clarification.
+4. follow_up_question: Required when should_follow_up is true—target key points the user hasn't covered, do not ask about already-answered content. Set to null when false.
 
-Format your response as JSON:
-{
-  "score": 8,
-  "feedback": "...",
-  "should_follow_up": false
-}`;
+Format as JSON:
+{"score":0-10,"feedback":"...","should_follow_up":true|false,"follow_up_question":"question or null"}
+
+Example (follow-up needed):
+{"score":6,"feedback":"Covered security and performance, missed dependencies.","should_follow_up":true,"follow_up_question":"How would you make the AI clarify task dependencies?"}
+
+Example (no follow-up):
+{"score":8,"feedback":"Complete answer.","should_follow_up":false,"follow_up_question":null}
+
+Bad example:
+{"should_follow_up":true,"follow_up_question":"Anything else?"}
+↑ User already provided details, vague "anything else" is unacceptable.`;
 
   let systemPrompt = isZh
     ? systemPromptZh
@@ -194,8 +214,7 @@ Format your response as JSON:
     systemPrompt += anchorText;
   }
 
-  const followupHints: string[] = answerKey?.followupHints || [];
-  const maxFollowUps = followupHints.length > 0 ? followupHints.length : 2;
+  const maxFollowUps = (currentQuestion as any).maxFollowUps ?? 2;
 
   const userContentText =
     typeof lastUserMessage.content === 'string'
@@ -259,10 +278,10 @@ Format your response as JSON:
     }
 
     let followupHintMsg: AIMessage | null = null;
-    if (shouldFollowUp && followupHints.length > 0) {
-      let hint = followupHints[Math.min(currentFollowUpCount, followupHints.length - 1)];
-      hint = hint.replace(/^如果.+?追问[：:]\s*/i, '').replace(/^[""「『]|[""」』]$/g, '');
-      followupHintMsg = new AIMessage(hint);
+    if (shouldFollowUp && result.follow_up_question && result.follow_up_question.trim()) {
+      followupHintMsg = new AIMessage(result.follow_up_question.trim());
+    } else if (shouldFollowUp) {
+      shouldFollowUp = false;
     }
 
     const feedbackMessage = new AIMessage(