Release v7.16.2

Fixed

• JsonSerializer now preserves the _derivedTypeId discriminator through both deserialization and serialization. Previously, deserializing a polymorphic value whose concrete type could not be resolved (e.g. its derived type was not yet registered) dropped the discriminator, so the value could never be re-resolved or round-tripped and serialized back with no type information. The discriminator now flows transparently in both directions.