feat(a2ui): add A2UI extension with v0.8/v0.9 support, schemas, and docs

Introduce the A2UI extension for declarative UI generation, including
support for both v0.8 and v0.9 protocol specs. Add A2UI content type
integration in A2A utils, along with schema definitions, catalog models,
and client extension improvements.

Enhance models with explicit defaults, field descriptions, and ConfigDict,
and improve typing and instance state handling across the extension.

Add schema conformance tests and align test structure.

Add and register A2UI documentation, including extension guide and
navigation updates.