Refactor tests to use _All_True model types throughout
Replaced all usages of legacy test model types (e.g., TestOrder, TestOrderItem, SharedTag, etc.) with new, feature-complete _All_True variants across SignalR test infrastructure, data sources, and service handlers. Updated all generic constraints, method signatures, and test data to use the new types. Added SharedTestBaseModels.cs and SharedTestOrderModels.cs to define abstract bases and concrete _All_True models with full serialization attributes. This enables more thorough and realistic serialization/deserialization testing and future extensibility.
- Support optional domain context in Toon serializer metadata (@meta section now includes "context" property if provided).
- Update internal and public APIs to accept and propagate domain context.
- Add overloads for Serialize, SerializeTypeMetadata, and SerializeMetadata to allow specifying domain context.
- Add AcBinarySerializerIIdReferenceTests and AcJsonSerializerIIdReferenceTests to verify IId-based reference deduplication and reference identity for both binary and JSON serializers.
- Tests cover both int and Guid IId scenarios, data integrity, and output size efficiency.
Loretta