[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.