优化 AI 设计任务失败提示

问题：客户端直接展示 policy_blocked 等内部错误码。

实现：统一转换为中文设计语言提示，并补充策略拦截场景回归测试。

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

@@ -482,6 +482,21 @@ describe('ImageCanvas Workspace-first design experience', () => {
     expect(confirmImageWorkspaceGenerationMock).not.toHaveBeenCalled();
   });
 
+  it('renders policy-blocked generation tasks with a user-facing design message', async () => {
+    fetchImageWorkspaceTasksMock.mockResolvedValue([{
+      ...taskFixture,
+      status: 'failed',
+      failureCode: 'policy_blocked',
+      resultAssets: [],
+    }]);
+
+    render(<MemoryRouter><ImageCanvas /></MemoryRouter>);
+
+    const alert = await screen.findByRole('alert');
+    expect(alert).toHaveTextContent('内容可能涉及版权或安全风险，请调整设计后重试');
+    expect(alert).not.toHaveTextContent('policy_blocked');
+  });
+
   it('renders a new task pushed by the design event stream without repeated polling', async () => {
     const queuedTask = {
       ...taskFixture,