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')

test/plugins/markdown/document_markdown_test.dart

@@ -57,6 +57,38 @@ void main() {
       expect(nodes[0].delta?.toPlainText(), 'This is the first line');
       expect(nodes[1].attributes['url'], 'https://example.com/image.png');
     });
+
+    // Regression test for https://github.com/AppFlowy-IO/AppFlowy/issues/8486
+    // documentToMarkdown() must not mutate the source document when processing
+    // nested (indented) list items.
+    test('documentToMarkdown does not orphan children of nested list items',
+        () {
+      final childNode = bulletedListNode(text: 'Child item');
+      final parentNode = bulletedListNode(
+        text: 'Parent item',
+        children: [childNode],
+      );
+      final document = Document(root: pageNode(children: [parentNode]));
+
+      // Verify initial structure: parent has 1 child.
+      expect(document.root.children.length, 1);
+      expect(document.root.children.first.children.length, 1);
+
+      final markdown = documentToMarkdown(document);
+
+      // The markdown output should be correct.
+      expect(markdown, '* Parent item\n\t* Child item\n');
+
+      // After conversion, the source document must be unchanged.
+      // Before the fix, convertNodes() called pageNode(children: nodes) which
+      // invoked unlink() on each child, removing them from their original parent.
+      expect(
+        document.root.children.first.children.length,
+        1,
+        reason:
+            'documentToMarkdown() must not remove children from the source document',
+      );
+    });
   });
 }