Support variant rewards when automatically adding BuyXGetY rewards

[kha333n]

A merchant picks a specific variant as a Buy X Get Y reward — which the admin
lets them do — and ticks "automatically add rewards". Every cart that qualifies
then dies with a 500.

What happens now

• Call to a member function first() on null in
  BuyXGetY::processAutomaticRewards(), thrown during cart calculation.
• It takes down anything that calculates the cart, so the shopper cannot view
  their basket, not just complete checkout.
• Product rewards and collection rewards both work. Only the variant reward,
  combined with the automatic-add option, fails.

What should happen

• A variant reward is added to the cart in the same way a product reward is.

Why

#2214 made variants selectable as rewards, and #2461 added a branch for
collections. The remaining else still assumes the reward is a Product and
reaches for its variants:

} else {
    $purchasable = $selectedRewardItem->variants->first();
}

A ProductVariant has no variants relation, so the property read returns
null and the ->first() on it is fatal. The null guard immediately below
never runs, because the error is thrown before it is reached.

The fix

Add a branch for a reward that is already purchasable, and use it directly.
ProductVariant implements Lunar\Base\Purchasable, and Product does not,
so the check separates the two cases without naming concrete models — the same
approach the existing collection branch takes.

Nothing else in the method changes: the existing product and collection paths
are untouched, and the reward still flows through the normal cart-line pipeline
afterwards.

Worth flagging: #2509 is currently rewriting this method to skip rewards that
cannot be fulfilled. The two changes do not overlap in intent, but whichever
lands second will want a rebase.

Tests

tests/core/Unit/DiscountTypes/BuyXGetYTest.php:

• can add an eligible variant reward when not in cart — the existing
  can add eligible products when not in cart scenario with the reward
  registered against the variant instead of the product. Fails on 1.x with
  Call to a member function first() on null at BuyXGetY.php:255, and
  asserts the same £12.00 total and single free item afterwards.