Store automatically added BuyXGetY reward lines using the morph map

[kha333n]

A reward line added automatically by a Buy X Get Y discount is written to
lunar_cart_lines with a different purchasable_type than every other line
holding the same product.

What happens now

• The reward line stores Lunar\Models\ProductVariant, while a line the
  shopper added stores product_variant.
• The same purchasable therefore appears under two different types in one
  cart.
• Any query that filters cart lines by morph type — reporting, an
  whereHasMorph, a custom pipeline stage — silently skips these lines.
• It survives into lunar_order_lines, so the inconsistency is persisted
  against the placed order too.

What should happen

• A reward line stores the same purchasable_type as any other line for that
  purchasable.

Why

Lunar registers a morph map at boot (ModelManifest::morphMap(), keyed by the
snake-cased class basename), so ProductVariant::getMorphClass() returns
product_variant. Every cart line in the codebase is created with
getMorphClass(), except this one:

$rewardLine = $cart->lines()->make([
    'purchasable_type' => get_class($purchasable),

get_class() bypasses the map and writes the concrete class name.

The fix

Use getMorphClass(), matching how cart lines are created everywhere else. It
also means a store that has extended ProductVariant gets the mapped alias
rather than its own class name written into the table.

Deliberately out of scope: with reward_qty greater than one this method also
creates one cart line per reward unit rather than a single line, because its
"is it already in cart?" check reads an in-memory $cart->lines the loop never
appends to. That is a real but separate defect, and fixing it here would make
this diff much harder to review. Also note #2509 is currently rewriting this
method for stock availability — no overlap in intent, but a rebase for whichever
lands second.

Tests

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

• can store an automatically added reward line using the morph map — an
  automatic reward on a qualifying cart, asserting the persisted
  purchasable_type. Fails on 1.x with:

  -'product_variant'
  +'Lunar\Models\ProductVariant'