fix(editor): align shouldPreserveNewLines between import and export

Import () was using shouldPreserveNewLines=true
but export () defaulted to false. This mismatch
caused each line in a multi-line body to become a separate ParagraphNode
on import, then export joined adjacent non-empty paragraphs with \n\n,
turning every \n into \n\n (e.g. aaa\nbbb\nccc → aaa\n\nbbb\n\nccc).

Fix: pass shouldPreserveNewLines=true to export too, so ParagraphNodes
are joined with just \n — consistent behavior in both directions.

Tests added:
- Multi-line body round-trips without phantom blank lines
- Leading blank lines are preserved through round-trip
- Intentional paragraph breaks (empty lines) are preserved

Author: koki-develop

src/components/molecules/MarkdownEditor/MarkdownEditor.tsx

@@ -290,7 +290,9 @@ export const MarkdownEditor = forwardRef<HTMLDivElement, MarkdownEditorProps>(
   ) {
     const handleChange = useCallback(
       (editorState: EditorState) => {
-        onChange(editorState.read(() => $convertToMarkdownString(MARKDOWN_TRANSFORMERS)));
+        onChange(
+          editorState.read(() => $convertToMarkdownString(MARKDOWN_TRANSFORMERS, undefined, true)),
+        );
       },
       [onChange],
     );

src/components/molecules/MarkdownEditor/__tests__/utils.tsx

@@ -48,7 +48,9 @@ export function $setMarkdown(editor: LexicalEditor, markdown: string): void {
 }
 
 export function $readMarkdown(editor: LexicalEditor): string {
-  return editor.getEditorState().read(() => $convertToMarkdownString(MARKDOWN_TRANSFORMERS));
+  return editor
+    .getEditorState()
+    .read(() => $convertToMarkdownString(MARKDOWN_TRANSFORMERS, undefined, true));
 }
 
 type RenderTestEditorOptions = {

src/components/molecules/MarkdownEditor/transformers.spec.ts

@@ -18,4 +18,22 @@ describe("MARKDOWN_TRANSFORMERS round-trip", () => {
     $setMarkdown(editor, "- one\n- two");
     expect($readMarkdown(editor)).toBe("- one\n- two");
   });
+
+  test("a multi-line body round-trips without phantom blank lines", () => {
+    const editor = createTestHeadlessEditor();
+    $setMarkdown(editor, "aaa\nbbb\nccc");
+    expect($readMarkdown(editor)).toBe("aaa\nbbb\nccc");
+  });
+
+  test("leading blank lines are preserved through round-trip", () => {
+    const editor = createTestHeadlessEditor();
+    $setMarkdown(editor, "\n\nbody");
+    expect($readMarkdown(editor)).toBe("\n\nbody");
+  });
+
+  test("intentional paragraph breaks (empty lines) are preserved", () => {
+    const editor = createTestHeadlessEditor();
+    $setMarkdown(editor, "aaa\n\nbbb");
+    expect($readMarkdown(editor)).toBe("aaa\n\nbbb");
+  });
 });