feat(cart): enhance item update functionality to support variant changes and improve cart editing policies

app/Domains/Purchase/Services/Checkout/SourceCartService.php

@@ -71,6 +71,57 @@ class SourceCartService
             ->update(['cantidad' => $quantity]);
     }
 
+    public function syncItemSelection(
+        Purchase $purchase,
+        int $catalogItemId,
+        ?int $previousVariantId,
+        int $newVariantId,
+        int $finalQuantity,
+    ): void {
+        $sourceCart = $this->findSourceCart($purchase);
+
+        if ($sourceCart === null) {
+            return;
+        }
+
+        /** @var CartItem|null $previousItem */
+        $previousItem = $sourceCart->items()
+            ->where('catalog_item_id', $catalogItemId)
+            ->where('variant_id', $previousVariantId)
+            ->lockForUpdate()
+            ->first();
+
+        /** @var CartItem|null $targetItem */
+        $targetItem = $sourceCart->items()
+            ->where('catalog_item_id', $catalogItemId)
+            ->where('variant_id', $newVariantId)
+            ->when($previousItem !== null, fn ($query) => $query->whereKeyNot($previousItem->getKey()))
+            ->lockForUpdate()
+            ->first();
+
+        if ($targetItem !== null) {
+            $targetItem->update(['cantidad' => $finalQuantity]);
+            $previousItem?->delete();
+
+            return;
+        }
+
+        if ($previousItem !== null) {
+            $previousItem->update([
+                'variant_id' => $newVariantId,
+                'cantidad' => $finalQuantity,
+            ]);
+
+            return;
+        }
+
+        $sourceCart->items()->create([
+            'catalog_item_id' => $catalogItemId,
+            'variant_id' => $newVariantId,
+            'cantidad' => $finalQuantity,
+        ]);
+    }
+
     public function finalize(Purchase $purchase): void
     {
         $sourceCart = $this->findSourceCart($purchase);