Release v7.9.1

Fixed

• Corrected target type resolution logic in DerivedTypes so that a known targetType is validated with IsAssignableTo and returned early, and the interface-based fallback only runs when targetType is null.
• TypeScript derivedType decorator now accepts an optional targetType, registering the decorated class against its base type for automatic discovery during deserialization.
• JsonSerializer deserialization now resolves derived types from both explicit field derivatives and types registered via DerivedType.getDerivedTypesFor, eliminating the need to list every subtype on every field.
• JsonSerializer serialization now writes the _type discriminator property automatically when the instance's constructor has a registered derived type identifier, ensuring derived type instances round-trip correctly.