feat(purchase): add expiration time calculations to PurchaseResource and implement related tests

app/Domains/Purchase/Resources/PurchaseResource.php

@@ -17,6 +17,8 @@ class PurchaseResource extends JsonResource
      */
     public function toArray(Request $request): array
     {
+        $serverTime = now();
+        $expiresAt = $this->stockReservation?->expires_at;
         $items = $this->resource->relationLoaded('items')
             ? $this->resource->getRelation('items')
             : collect();
@@ -48,7 +50,11 @@ class PurchaseResource extends JsonResource
             'created_at' => $this->created_at,
             'status' => $this->status,
             'payment_method' => $this->payment_method,
-            'expires_at' => $this->stockReservation?->expires_at,
+            'expires_at' => $expiresAt,
+            'expires_in_seconds' => $expiresAt === null
+                ? null
+                : max(0, $expiresAt->getTimestamp() - $serverTime->getTimestamp()),
+            'server_time' => $serverTime,
             'dni' => $this->dni,
             'transfer_payer_dni' => $this->transfer_payer_dni,
             'telefono' => $this->telefono,