The Tiptap Mention extension's createInlineMarkdownSpec serializes
mentions as shortcodes [@ id="..." label="..."] — the .extend()
renderMarkdown override may not reliably take effect.
Added a robust fallback: post-process the editor's markdown output
by replacing shortcodes with [@Label](mention://type/id) using the
Tiptap JSON document for type info. Also preprocess stored shortcodes
in the Markdown renderer for backward compatibility.
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