* test: add checkout shipping method selection regression test
Verifies that the checkout mutation respects the shipping method from
the input and doesn't silently fall back to the WooCommerce default
(cheapest) rate.
Resolves#927
* fix: remove duplicate mutation registrations in type registry
Removed duplicate register_mutation() calls for checkout, coupon
(create/update/delete), and review (write/update/delete_restore)
mutations that were introduced during rebase.
