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.
Split SignalRDataSourceTests into multiple focused files and introduce an abstract base test class for comprehensive coverage. Add concrete test classes for all serializer and collection type combinations. Move data source implementations to separate files. Test coverage now includes edge cases, interface compliance, and advanced scenarios.
Loretta