New validation against duplicate handles in productCreate, productUpdate, and productSet mutation inputs

As of the 2025-01 version of the Admin GraphQL API, the handle field in productInput for productCreate and productUpdate, as well as in productSetInput for productSet will be validated for uniqueness. This means that you will no longer be able to input a duplicate handle. This change does not affect existing behaviour when handle is not provided as input.

This enhancement ensures that there are no collisions when creating or updating product handles.