[FEATURE]: Add platform-agnostic conformance tests for a2ui-core libraries.

Currently, our inference libraries have conformance tests for some features, which verify inputs and outputs. The expected inputs and outputs are encoded in YAML, with a description of how the test rig should be configured to use them.

We should do the same thing for the core libraries. e.g. to verify:
- Processing of messages is correct
- State of models is updated correctly - e.g. surfaceModel, componentModel etc, but also the node tree once it's implemented.
- The expected errors are emitted correctly
- Data model lookups work as expected with paths etc
- Functions work as expected, including nesting, functions that expose streams of data from the client etc.

This would go a long way towards making sure our libraries are consistent with each other.

Look at existing conformance tests for inference libraries to understand the YAML format and test rig configuration. Examine core libraries like surfaceModel and componentModel to identify key behaviors to test. The goal is to create tests that verify message processing, state updates, error emission, data model lookups, and function behavior across libraries.

Written by the indexing model from the issue text.