[Bug] The `PersistableModelCreateCore` method is changing the return type and it is not allowed

[ArcturusZhang]

The code on main builds with this failure:

src\Generated\Models\ModelWithBaseModelWithRequired.Serialization.cs(114,59): error CS8830: 'ModelWithBaseModelWith
Required.PersistableModelCreateCore(BinaryData, ModelReaderWriterOptions)': Target runtime doesn't support covariant re
turn types in overrides. Return type must be 'BaseModelWithRequired' to match overridden member 'BaseModelWithRequired.
PersistableModelCreateCore(BinaryData, ModelReaderWriterOptions)'