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.
- Add overflow/corruption guards to string (de)serialization (writer/reader now throw on invalid lengths)
- Remove dead string serialization methods per BINARY_TODO.md audit
- Update BINARY_TODO.md with closure/resolution for H2Q6, O7G2, V4N5, and related entries
- Add MaxStringCharLength constant and update marker reservations in BinaryTypeCode
- Simplify string cache ASCII verification in deserializer
- Add SGen/Runtime round-trip compatibility tests for large/deep data
- Minor code modernization and style improvements
Loretta