fix: shuffle choice options per session + clean followup hints

- Options shuffled with correctAnswer remapped at session creation
- Followup hints strip conditional prefix (如果只说了XX，追问：)

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

@@ -254,7 +254,8 @@ Format your response as JSON:
 
     let followupHintMsg: AIMessage | null = null;
     if (shouldFollowUp && followupHints.length > 0) {
-      const hint = followupHints[Math.min(currentFollowUpCount, followupHints.length - 1)];
+      let hint = followupHints[Math.min(currentFollowUpCount, followupHints.length - 1)];
+      hint = hint.replace(/^如果[^，,]*[，,]\s*追问[：:]\s*/i, '').replace(/^如果[^。]*[。]\s*/i, '');
       const hintLabel = isZh
         ? `让我帮你完善一下：${hint}`
         : isJa