Refine SQL output formatting in prompts to enforce pure text SQL without Markdown code fences. Allow inline comments using -- and update few-shot selector to reflect these changes in output presentation. Enhance clarity on SQL output requirements for better adherence to standards.

backend/utils/fewshot_selector.py

@@ -491,7 +491,9 @@ class FewShotSelector:
             lines.append(f"## {ex.qid}. {ex.question_zh[:50]}")
             lines.append(f"评分: {ex.rating}/10")
             lines.append(f"标签: {', '.join(ex.tags)}")
-            lines.append(f"```sql\n{ex.sql}\n```\n")
+            lines.append("SQL（纯文本，允许 -- 注释；禁止 ``` 围栏）：")
+            lines.append(ex.sql)
+            lines.append("")
 
         return "\n".join(lines)