fix(core): in page mode, AI replace selection did not correctly replace the selected text (#9179)

Fix issue [BS-1980](https://linear.app/affine-design/issue/BS-1980).

packages/frontend/core/src/blocksuite/presets/ai/utils/editor-actions.ts

@@ -105,6 +105,7 @@ export const replace = async (
   );
 
   if (textSelection) {
+    host.std.command.exec('deleteText', { textSelection });
     const { snapshot, job } = await markdownToSnapshot(content, host);
     await job.snapshotToSlice(
       snapshot,