错题本api 返回字段优化

app/Services/MistakeBookService.php

@@ -705,12 +705,12 @@ class MistakeBookService
             'next_review_at' => $mistake->next_review_at?->toISOString(),
             'error_type' => $mistake->error_type,
             'error_type_label' => $mistake->error_type_label,
-            // 【移除】kp_ids、source、source_label、knowledge_point、explanation 字段
+            // 【移除】kp_ids、source、source_label、knowledge_point 字段
             // 'kp_ids' => $mistake->kp_ids,
             // 'source' => $mistake->source,
             // 'source_label' => $mistake->source_label,
             // 'knowledge_point' => $mistake->knowledge_point,
-            // 'explanation' => $mistake->explanation,
+            'explanation' => $mistake->explanation,
             'skill_ids' => $mistake->skill_ids,
             'difficulty' => $mistake->difficulty,
             'difficulty_level' => $mistake->difficulty_level,