fix: unit test and integration test

frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/ai/operations/ai_writer_node_extension.dart

@@ -72,8 +72,11 @@ extension AiWriterNodeExtension on EditorState {
       }
     }
 
+    // use \n\n as line break to improve the ai response
+    // using \n will cause the ai response treat the text as a single line
     final markdown = await customDocumentToMarkdown(
       Document.blank()..insert([0], slicedNodes),
+      lineBreak: '\n\n',
     );
 
     // trim the last \n if it exists

frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/base/markdown_text_robot.dart

@@ -424,6 +424,7 @@ class MarkdownTextRobot {
         firstMarkdownDelta != null) {
       final startIndex = selection.startIndex;
       final length = delta.length - startIndex;
+
       transaction
         ..deleteText(firstNode, startIndex, length)
         ..insertTextDelta(firstNode, startIndex, firstMarkdownDelta);
@@ -439,7 +440,9 @@ class MarkdownTextRobot {
         lastMarkdownNode != null &&
         lastMarkdownDelta != null) {
       final endIndex = selection.endIndex;
+
       transaction.deleteText(lastNode, 0, endIndex);
+
       // if the last node is same as the first node, it means we have replaced the
       // selected text in the first node.
       if (lastMarkdownNode.id != firstMarkdownNode?.id) {
@@ -457,8 +460,9 @@ class MarkdownTextRobot {
     }
 
     // step 4
-    if (nodes.length > 2) {
+    final length = nodes.length - 2;
-      final middleNodes = nodes.skip(1).take(nodes.length - 2).toList();
+    if (length > 0) {
+      final middleNodes = nodes.skip(1).take(length).toList();
       transaction.deleteNodes(middleNodes);
     }
 

frontend/appflowy_flutter/lib/shared/markdown_to_document.dart

@@ -27,6 +27,7 @@ Future<String> customDocumentToMarkdown(
   Document document, {
   String path = '',
   AsyncValueSetter<Archive>? onArchive,
+  String lineBreak = '',
 }) async {
   final List<Future<ArchiveFile>> fileFutures = [];
 
@@ -41,7 +42,7 @@ Future<String> customDocumentToMarkdown(
   try {
     markdown = documentToMarkdown(
       document,
-      lineBreak: '\n\n',
+      lineBreak: lineBreak,
       customParsers: [
         const MathEquationNodeParser(),
         const CalloutNodeParser(),

frontend/appflowy_flutter/test/bloc_test/ai_writer_test/ai_writer_bloc_test.dart

@@ -375,7 +375,7 @@ void main() {
       await blocResponseFuture();
       bloc.runResponseAction(SuggestionAction.accept);
       await blocResponseFuture();
-      expect(editorState.document.root.children.length, 1);
+      expect(editorState.document.root.children.length, 2);
       expect(
         editorState.getNodeAtPath([0])!.delta!.toPlainText(),
         'Hello World',