fix: prevent documentToMarkdown from mutating source document (#1184)

* 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.

lib/src/plugins/markdown/encoder/document_markdown_encoder.dart

@@ -34,7 +34,9 @@ class DocumentMarkdownEncoder extends Converter<Document, String> {
     List<Node> nodes, {
     bool withIndent = false,
   }) {
-    final result = convert(Document(root: pageNode(children: nodes)));
+    final result = convert(
+      Document(root: pageNode(children: nodes.map((n) => n.deepCopy()))),
+    );
     if (result.isNotEmpty && withIndent) {
       return result
           .split('\n')