fix: drag mind map root node should layout in real time (#9252)

Fixes [BS-2062](https://linear.app/affine-design/issue/BS-2062/拖拽整个-mind-map-还是希望和之前一样，整个思维导图跟手)

blocksuite/affine/model/src/elements/mindmap/mindmap.ts

@@ -624,6 +624,15 @@ export class MindmapElementModel extends GfxGroupLikeElementModel<MindmapElement
     return this._nodeMap.get(id) ?? null;
   }
 
+  getNodeByPath(path: number[]): MindmapNode | null {
+    let node: MindmapNode | null = this._tree;
+    for (let i = 1; i < path.length; i++) {
+      node = node?.children[path[i]];
+      if (!node) return null;
+    }
+    return node;
+  }
+
   getParentNode(id: string) {
     const node = this.children.get(id);