XMarkdown Mermaid、CodeHighlighter希望可以支持自定义渲染头部、内容

### 需求动机

目前XMarkdown图表、代码组件的头部区域只能显示、隐藏，希望可以加入自定义的render展示同的元素内容

### 提议的 API 是什么样的？

_No response_

Start by locating the XMarkdown, Mermaid, and CodeHighlighter component entry points and reading how their header and content are currently shown or hidden. Clarify the proposed rendering API and expected customization boundaries with maintainers. Done means both components support the agreed custom rendering behavior with coverage for the relevant display cases.

Written by the indexing model from the issue text.