fix: account for discounts applied by bpn api in validation request

src/BusProNet/Model/BookingResponse.php

@@ -102,4 +102,45 @@ class BookingResponse
             2
         );
     }
+
+    /**
+     * Gets API-applied discounts that cannot be predicted by the local price calculator.
+     *
+     * The BusProNet API may apply automatic discounts based on business rules that
+     * are not known to the local application, such as:
+     * - ERM/GRU: Gruppenrabatt (group discount for large bookings)
+     * - Other ERM subtypes: Various automatic discounts
+     *
+     * These discounts appear as negative price items and must be accounted for when
+     * comparing the API total price against the locally calculated price.
+     *
+     * Note: Voucher discounts (AKTION, KULANZGUTSCHEIN) are handled separately via
+     * getVoucherDiscountFromPrices() as they are user-initiated, not automatic.
+     *
+     * @return array<PriceItem> Price items representing API-applied automatic discounts
+     */
+    public function getApiAppliedDiscounts(): array
+    {
+        return array_filter(
+            $this->priceItems,
+            fn (PriceItem $item) => $item->totalPrice < 0
+                && false === \in_array($item->type, ['AKTION', 'KULANZGUTSCHEIN'], true)
+        );
+    }
+
+    /**
+     * Gets total discount from API-applied automatic discounts.
+     *
+     * Returns the absolute sum of negative price items that represent automatic
+     * discounts applied by the API (excluding voucher discounts which are handled separately).
+     */
+    public function getApiAppliedDiscountTotal(): float
+    {
+        $discount = 0.0;
+        foreach ($this->getApiAppliedDiscounts() as $item) {
+            $discount += abs($item->totalPrice);
+        }
+
+        return round($discount, 2);
+    }
 }