Align chat renderer fallbacks

[sdirix]

Bug Description:

• The chat-view-tree-widget checks whether any of the response part renderers matches to the parts to render. If there is none, it will log and render an error.
• The horizontal-layout-part-renderer also does dispatching, however it does not have such a check. Instead it will take the first renderer, which is an arbitrary choice and may lead to runtime errors.

Steps to Reproduce:

1. In an agent, return a HorizontalLayoutChatResponseContentImpl with a response part which can't be handled by any of the renderers.

Expected behavior

Both cases should behave the same, at minimum logging a debug message and making sure that there is no runtime error, for example by falling back to a default renderer or rendering some fallback content.