feat: refactor cart and purchase handling to support polymorphic buyable types and improve code readability

tests/Feature/Integration/TelepagosWebhookTest.php

@@ -113,7 +113,8 @@ class TelepagosWebhookTest extends TestCase
 
         $this->assertDatabaseHas('compra_items', [
             'compra_id' => $matchingPurchase->id,
-            'producto_variante_id' => $variant->id,
+            'buyable_type' => ProductVariant::class,
+            'buyable_id' => $variant->id,
             'cantidad' => 1,
             'total' => 50,
         ]);
@@ -201,7 +202,7 @@ class TelepagosWebhookTest extends TestCase
             'status' => 'active',
         ]);
 
-        $cart->addItem($variantId, $quantity);
+        $cart->addItem(ProductVariant::class, $variantId, $quantity);
 
         /** @var CheckoutService $checkoutService */
         $checkoutService = app(CheckoutService::class);