* fix: prevent documentToMarkdown from mutating source document
convertNodes() was calling pageNode(children: nodes) which triggered
Node's constructor to unlink() each child from its original parent,
orphaning nested list items in the source document.
Fix by deep-copying nodes before passing them to pageNode().
Fixes https://github.com/AppFlowy-IO/AppFlowy/issues/8486
* chore: add regression test for issue #8486
Verifies that documentToMarkdown() does not orphan children of nested
list items in the source document after conversion.
301d0fa579