修复视频首帧选择弹窗交互

问题：选择作品图片后需等待 Agent 整轮响应，弹窗长时间不关闭。
修复：选择后立即关闭弹窗并继续提交 Asset ID，失败信息转到主对话区；补充慢响应回归测试。

tests/unit/image-canvas-page.test.tsx

@@ -570,6 +570,8 @@ describe('ImageCanvas Workspace-first design experience', () => {
   });
 
   it('submits a successful work Asset id when it is selected as the video first frame', async () => {
+    const pendingSelection = deferred<DesignWorkspace>();
+    sendImageWorkspaceMessageMock.mockReturnValueOnce(pendingSelection.promise);
     fetchImageWorkspaceProjectMock.mockResolvedValueOnce({
       ...workspaceFixture(),
       turnRevision: 2,
@@ -603,6 +605,12 @@ describe('ImageCanvas Workspace-first design experience', () => {
       expect.stringMatching(/^turn-/),
       ['asset-one'],
     ));
+    expect(screen.queryByRole('dialog', { name: '选择视频首帧' })).not.toBeInTheDocument();
+
+    await act(async () => {
+      pendingSelection.resolve(workspaceFixture());
+      await pendingSelection.promise;
+    });
   });
 
   it('uploads a local first-frame image and submits its Asset id to the Agent', async () => {