Check for duplicate product codes from different package IDs

> I believe there should be some kind of PR verification in the winget-pkgs repository to ensure that a package whose Product Code is already used by some other existing package cannot be introduced.
>
> Tagging @denelon for visibility.

_Originally posted by @felipecrs in [#267669](https://github.com/microsoft/winget-pkgs/issues/267669#issuecomment-2997223144)_

The issue does not name files, tests, or an entry point. Start by tracing how pull-request validation works in the winget-pkgs repository and how package IDs and Product Codes are represented; done means duplicate Product Codes across different package IDs are detected and rejected during verification.

Written by the indexing model from the issue text.