feat: correct determination of inquiry status and conditional voucher field display

src/BusProNet/Constants.php

@@ -52,6 +52,11 @@ final class Constants
     public const BOOKING_TYPE_INQUIRY = 'Anfrage';
     public const BOOKING_TYPE_BOOKING = 'Buchung';
 
+    // Booking status codes from buchungstatusmoeglich attribute
+    public const BOOKING_STATUS_FREE = 'F';
+    public const BOOKING_STATUS_INQUIRY = 'A';
+    public const BOOKING_STATUS_OPEN = 'O'; // Optionsbuchung - not relevant for current implementation
+
     // Payment methods
     public const PAYMENT_METHOD_TRANSFER = 'transfer';
     public const PAYMENT_METHOD_DEBIT = 'debit';