fix: avoid pre-sanitizing markdown source

[A-Words]

What this PR does / why we need it?

关联 Issue: #11998
目标: 修复 Markdown 代码块中 </> 被过度防御转义的问题。
背景: 组件此前在 Markdown 渲染前先对原始内容做 DOMPurify.sanitize，会影响代码块文本显示。

Summary of your change

将 mkdown-editor 的输入从 v-model="sanitizedReadMe" 改为 :model-value="props.content"，不再预处理 Markdown 源文本。
新增 sanitizeHtml 并通过 :sanitize="sanitizeHtml" 交给 MdEditor 在 HTML 阶段进行消毒。

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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[lan-yonghui]

/lgtm