fix: consider booking status O correctly

src/BusProNet/Constants.php

@@ -57,7 +57,7 @@ final class Constants
     // 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
+    public const BOOKING_STATUS_OPEN = 'O'; // Optionsbuchung
 
     // Payment methods
     public const PAYMENT_METHOD_TRANSFER = 'transfer';