Prevent combining carts from different stores

Thank you for opening the issue and creating a merge request, hopefully someone will get a chance to review it.

BTW once you are finished with your work on an issue please set the "assigned" field to "Unassigned", that lets others know that you're done and that then can then take a look. Thank you.

Tested MR, worked fine.

Would there ever be a situation where you would want to combine carts from different stores? I can't quite remember the data model, but doesn't make much sense to even attempt to merge them, does it?

Thanks for your work on this. I've checked, and of course, you are right, orders belong to a single store, so it doesn't make much sense to combine carts that don't have the same store.

Let's get some tests in for this while we're here, and then this can get committed.