fix: shuffle bank questions + grader LLM error resilience

- selectQuestions: shuffle final result for random question order
- grader: wrap LLM invoke in try-catch, default score 5 on failure
- grader: inner try-catch for JSON parse errors, graceful fallback

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

@@ -201,6 +201,7 @@ Format your response as JSON:
   console.log('[GraderNode] Question:', currentQuestion?.questionText?.substring(0, 100));
   console.log('[GraderNode] Target dimension:', currentQuestion?.dimension);
 
+  try {
   const response = await model.invoke([
     new SystemMessage(systemPrompt),
     new HumanMessage(userContentText),
@@ -279,14 +280,29 @@ Format your response as JSON:
         ? currentQuestionIndex
         : currentQuestionIndex + 1,
     } as any;
-  } catch (error) {
-    console.error('Failed to parse grade from AI response:', error);
+  } catch (parseError) {
+    console.error('[GraderNode] Failed to parse grade:', parseError);
+    const scoreLabel = isZh ? '得分' : isJa ? 'スコア' : 'Score';
+    const fallbackMsg = new AIMessage(`${scoreLabel}: 5/10\n\n评分解析失败，默认给5分。`);
     return {
-      feedbackHistory: [
-        new AIMessage("I had some trouble grading that, but let's move on."),
-      ],
-      currentQuestionIndex: currentQuestionIndex + 1,
+      feedbackHistory: [fallbackMsg],
+      scores: { [currentQuestion.id || currentQuestionIndex.toString()]: 5 },
       shouldFollowUp: false,
+      followUpCount: 0,
+      currentQuestionIndex: currentQuestionIndex + 1,
+    } as any;
+  }
+  } catch (error) {
+    console.error('[GraderNode] LLM grading failed:', error);
+    const scoreLabel = isZh ? '得分' : isJa ? 'スコア' : 'Score';
+    const feedbackLabel = isZh ? '反馈' : isJa ? 'フィードバック' : 'Feedback';
+    const fallbackMsg = new AIMessage(`${scoreLabel}: 5/10\n\n${feedbackLabel}: 评分服务暂时不可用，默认给5分。`);
+    return {
+      feedbackHistory: [fallbackMsg],
+      scores: { [currentQuestion.id || currentQuestionIndex.toString()]: 5 },
+      shouldFollowUp: false,
+      followUpCount: 0,
+      currentQuestionIndex: currentQuestionIndex + 1,
     } as any;
   }
 };

server/src/assessment/services/question-bank.service.ts

@@ -588,7 +588,7 @@ export class QuestionBankService {
     this.logger.log(
       `[selectQuestions] Selected ${selected.length} questions from bank ${bankId}`,
     );
-    return selected;
+    return this.shuffleArray(selected);
   }
 
   async batchReviewItems(