Implement the concept of "Composite" components

Imagine if you could define a `CompositeComponent` which is implemented in A2UI, using other components. This concept could exist entirely within the agent inference libraries. When the LLM returns a layout, the agent library can then convert the Composite Components to traditional Primitive Components before sending them to the client.

This provides a way to create higher level abstractions for agents without changing a client implementation.

Look at the agent inference libraries in the codebase to understand how layouts are currently generated and sent to the client. Identify where primitive components are defined and how a composite component abstraction could be inserted into that flow. The goal is to design a CompositeComponent type that the LLM can return, and a conversion layer that expands it into primitives before client delivery.

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