Fix cart retrieval to prefer unmerged carts (and clarify active scope grouping)

[vencious]

Summary

Fixes an issue where an authenticated user could have a previously-merged cart selected during cart resolution, leading to unexpected “missing” items after login/logout cycles. Also makes the Cart::active() scope grouping explicit for readability/intent.

Why is this needed?

When a user has carts that were merged into a newer cart, selecting ->active()->first() may return a merged/source cart depending on ordering and query shape. We now prefer the latest unmerged active cart.

Changes

• In CartSessionManager, prefer the latest unmerged active cart when resolving an authenticated user's cart.
• Make the Cart::active() scope grouping explicit (readability/intent).
• Add regression tests covering merged cart selection and user-context active cart querying.

Fixes #2397

[alecritson]

Thanks @vencious Can you quickly update the tests so the comments are in English otherwise we're unsure of what's happening :)

[vencious]

Sorry about that — I’ve updated and pushed: removed the non-English comments and clarified the test intent via naming/assertions.