🐛 fix(ai): 修正 SQL 代码块 Markdown 换行渲染

- 为 AI markdown 渲染补充 fenced code block 预处理
- 修正 opening/closing fence 缺少换行时的代码块解析失败
- 补充回归测试并更新 issue backlog 记录

Fixes #369

frontend/src/utils/aiMarkdown.test.ts

@@ -0,0 +1,11 @@
+import { describe, expect, it } from 'vitest';
+
+import { normalizeAiMarkdown } from './aiMarkdown';
+
+describe('normalizeAiMarkdown', () => {
+  it('inserts a missing newline after the fenced code language marker', () => {
+    expect(normalizeAiMarkdown('```sqlSELECT COUNT(*) AS order_count\nFROM customer_order;\n```')).toBe(
+      '```sql\nSELECT COUNT(*) AS order_count\nFROM customer_order;\n```',
+    );
+  });
+});

frontend/src/utils/aiMarkdown.ts

@@ -0,0 +1,13 @@
+export const normalizeAiMarkdown = (content: string): string => {
+  let text = String(content || '').replace(/\r\n/g, '\n');
+  const knownFenceLanguages = [
+    'sql', 'mermaid', 'json', 'javascript', 'typescript', 'ts', 'js', 'tsx', 'jsx',
+    'bash', 'sh', 'shell', 'python', 'py', 'go', 'java', 'yaml', 'yml', 'html', 'css',
+    'xml', 'markdown', 'md', 'text', 'plaintext', 'vue', 'php', 'ruby', 'rust', 'toml',
+    'ini', 'diff',
+  ];
+  const fencePattern = new RegExp(`(^|\\n)\`\`\`(${knownFenceLanguages.join('|')})([^\\n])`, 'gi');
+  text = text.replace(fencePattern, '$1```$2\n$3');
+  text = text.replace(/([^\n])```(?=\n|$)/g, '$1\n```');
+  return text;
+};