[FEATURE]: Add initial conformance tests to the a2ui core SDK

The agent SDK currently has platform agnostic conformance tests.

We should add similar conformance tests to the core framework.

Let's start by adding a suite for the data model, e.g. conforming that updateing the data model at different paths etc should work correctly for all edge cases.

This can replace the existing unit tests for the data model in many cases.

1. Research how the framework currently works for the agent SDK
2. Plan how to translate that to the core SDK, including where it should be described in blueprints and readmes etc so that coding agents can discover and manage it. perhaps there should be a skill explaining how our conformance tests work overall. We may want to do some changes to repo structure, e.g. add a /conformance folder with the conformance test definitions and YAML sources of truth
3. Define some tests cases for data model in the core SDK
4. Hook up the conformance tests to the Python and TS data model implementations for now

Please come up with detailed plan for all of this before doing the implementation.

Untersuche die vorhandenen Conformance-Tests des agent SDK, um ihre Struktur zu verstehen. Plane, wie sie an das Datenmodell des core SDK angepasst werden können, und berücksichtige dabei, wo Definitionen in einem /conformance-Ordner platziert werden und wie die Integration mit den Python- und TypeScript-Implementierungen erfolgen soll. Die Arbeit umfasst Recherche, architektonische Planung und die Definition von Tests vor der Implementierung.

Vom Indexierungsmodell aus dem Issue-Text verfasst.