Match cart lines on the purchasable type when merging carts

[kha333n]

A shopper puts four gift cards in their basket as a guest, then logs in. The
gift cards are gone, and the one shampoo they already had is now five.

Fixes #2682.

The same code is unchanged on 2.x, so this wants a forward port.

What happens now

• Merging a guest cart into an account cart decides two lines are the same item
  from the purchasable id and the meta.
• Ids of different purchasable types are independent sequences, so gift card 1
  and product variant 1 both exist. Nothing coordinates them, and nothing needs
  to.
• When they collide, the source line is never created in the target cart and an
  unrelated line absorbs its quantity.
• The source cart is stamped merged_id immediately after, so the lost item
  cannot be recovered. No exception, nothing in the log.

What should happen

• Two lines are the same item only when the type and the id both match.

Why

MergeCart compares the id and the meta, and never the type:

return $targetLine->purchasable_id == $line->purchasable_id &&
json_encode($targetLine->meta) == json_encode($line->meta);

GetExistingCartLine — Lunar's other answer to "is this the same line?" —
does compare it, via wherePurchasableType(). The two disagree about what makes
two lines equal, and only one of them can be right.

The fix

Compare purchasable_type as well, which is the comparison
GetExistingCartLine already makes. Nothing else changes: the branch that
creates a missing line already writes the type correctly, so once the match is
right the rest of the method behaves.

Tests

tests/core/Unit/Actions/Carts/MergeCartTest.php:

• can merge lines of different purchasable types that share an id — an
  account cart with one variant, a guest cart with four gift cards carrying the
  same id, merged. Fails on 1.x with:

  -    0 => 'Lunar\Tests\Core\Stubs\TestGiftCard x4'
  -    1 => 'product_variant x1'
  +    0 => 'product_variant x5'
  

The defect is only observable with two purchasable types in play, and the suite
has no Eloquent purchasable — TestPurchasable is a plain class and cannot be
the target of a morph. So the test adds TestGiftCard, a minimal model
implementing Purchasable, and creates its table in the test. It is the sort of
custom purchasable the docs invite — a gift card, a bundle, a subscription.

[github-actions]

Thanks for the PR! I couldn't find a linked issue or Discussion in the description.

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