refactor(inventory): update stock validation rules and enforce stock_difference requirement

app/Domains/Ticketing/FiestaFutbolInfantil/Services/MerchandiseService.php

@@ -245,7 +245,7 @@ class MerchandiseService
                 ...$variant,
                 'color' => $color->value,
                 'size' => $size->value,
-                'stock' => (int) $variant['stock'],
+                ...(array_key_exists('stock', $variant) ? ['stock' => (int) $variant['stock']] : []),
             ];
         })->all();
     }
@@ -369,8 +369,10 @@ class MerchandiseService
             ->firstOrFail();
 
         $variant->update(['precio' => $data['price']]);
-        $stockDifference = (int) ($data['stock_difference'] ?? ($data['stock'] - $inventory->availableStock()));
-        $inventory->adjustAvailableStock($stockDifference, idempotencyKey: $stockAdjustmentId);
+        $inventory->adjustAvailableStock(
+            (int) $data['stock_difference'],
+            idempotencyKey: $stockAdjustmentId,
+        );
         $this->syncDefinitions($variant, $itemAttributes, $data);
     }