Preserve union metadata when back compat preserves a property type

Uncovered as part of this [regen](https://github.com/Azure/azure-sdk-for-net/pull/62649). When applying back compat for a property type, we are not applying any current union variant types on the restored BinaryData type.

started work: https://github.com/microsoft/typespec/pull/11839

Start by reviewing the regeneration context in Azure/azure-sdk-for-net#62649 and the ongoing work in microsoft/typespec#11839. Trace the back-compat handling for a property type and its restored BinaryData type; done means current union variant types and metadata are preserved.

Written by the indexing model from the issue text.