fix: correct gateway isActive mapping and improve refund error handling

This fixes the 'is_active' column mapping in Gateway model and adds a null check for transaction.gateway in the PaymentService refund method.

Author: gabaoun

app/models/gateway.ts

@@ -8,7 +8,7 @@ export default class Gateway extends BaseModel {
   @column()
   declare name: string
 
-  @column()
+  @column({ columnName: 'is_active' })
   declare isActive: boolean
 
   @column()

app/services/payment_service.ts

@@ -87,7 +87,7 @@ export class PaymentService {
     }
 
     const instance = this.gatewayInstances[transaction.gateway?.name || '']
-    if (!instance) {
+    if (!instance || !transaction.gateway) {
       throw new Error('Gateway implementation not found for refund')
     }