Function create_payable_appointment 

pub fn create_payable_appointment(
    adapter: &dyn DatabaseOperations,
    config: &DbBackendConfig,
    schema: &AppSchema,
    request: &AppointmentCreate<'_>,
    payment: AppointmentPayment<'_>,
) -> Result<i64, DatabaseError>

Reserves an unpaid slot atomically before creating its provider session.