Description:
Currently, Mews does not support applying voucher codes to service promotions linked to a stay service in the Internet Booking Engine (IBE). While vouchers work correctly for standalone bookable services (e.g., parking rates), they are ignored when the same service is added as a service promotion to a stay service.

Use Case:
We frequently offer our corporate guests customized or free parking rates. To streamline this process, we need the ability to apply voucher codes to service promotions, ensuring:
-Automatic rate adjustments based on the voucher.
- Accurate inventory management (via bookable services).
- Self-service booking for guests through the IBE, without manual intervention.

Current Limitation:
Vouchers only work for standalone bookable services, not for service promotions within a stay service. This forces us to use manual workflows or bypass the IBE, which is inefficient.

Proposed Solution:
Enable voucher codes to apply to service promotions in the IBE, similar to how they currently work for standalone bookable services.