fix: 代码块 [object Object] — rehype-highlight 替换 span 后 String() 失败

根因: rehype-highlight 将代码文本替换为 <span class='hljs-*'> 元素后，
children 变成 React 元素数组, String(children) → '[object Object]'

修复: extractTextContent 递归提取纯文本，绕过语法高亮 span

src/components/chat/AssistantMessage.tsx

@@ -118,7 +118,8 @@ export function AssistantMessage({ message, isStreaming, streamContent }: Assist
                   components={{
                     code({ className, children, ...props }) {
                       const match = /language-(\w+)/.exec(className ?? '');
-                      const codeStr = String(children).replace(/\n$/, '');
+                      // rehype-highlight 会把代码替换为 <span> 高亮元素，需提取纯文本
+                      const codeStr = extractTextContent(children).replace(/\n$/, '');
                       if (!match) return <code className={className} {...props}>{children}</code>;
                       return <CodeBlock language={match[1]} code={codeStr} />;
                     },
@@ -185,3 +186,15 @@ function CodeBlock({ language, code }: { language: string; code: string }): Reac
     </Box>
   );
 }
+
+/** 从 React children（可能含高亮 span）递归提取纯文本 */
+function extractTextContent(children: React.ReactNode): string {
+  if (typeof children === 'string') return children;
+  if (typeof children === 'number') return String(children);
+  if (!children) return '';
+  if (Array.isArray(children)) return children.map(extractTextContent).join('');
+  if (typeof children === 'object' && 'props' in children) {
+    return extractTextContent((children as React.ReactElement).props.children);
+  }
+  return '';
+}