[renderers] Ensure "Graceful Degradation" works.

Ensure the current renderers support [graceful degradation](https://a2ui.org/concepts/catalogs/#graceful-degradation) of unknown catalog components:

> If an Agent uses a new component/property on an older client, the client MUST handle it gracefully (e.g., ignoring it or rendering a text fallback or a "Not Supported" placeholder) rather than crashing. The client may also report a validation error back to the agent, allowing the agent to self-correct and downgrade the UI automatically.

We need to ensure there's sufficient **testing** for this feature; and that our renderers don't crash when receiving an unexpected widget, AND report an error back to the agent to let it know that something's not rendering correctly.

查看代码库中的渲染器，了解它们如何处理未知的目录组件。编写测试来模拟接收到意外 widget 的情况，并验证渲染器不会崩溃，能够优雅地降级（例如显示 fallback），并向 agent 报告错误。检查现有的渲染器测试套件，看看是否已有类似测试，以及应该在哪里添加新测试。

由索引模型根据 Issue 内容生成。