[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.

Look at the renderers in the codebase to understand how they handle unknown catalog components. Write tests that simulate receiving an unexpected widget and verify the renderer does not crash, gracefully degrades (e.g., shows a fallback), and reports an error back to the agent. Check existing test suites for renderers to see if similar tests exist and where to add new ones.

Written by the indexing model from the issue text.