fix: stop offering unbookable services in the booking edit flow

src/BusProNet/Model/Booking.php

@@ -143,6 +143,31 @@ class Booking
         });
     }
 
+    /**
+     * Checks whether a participant already holds a service in this booking.
+     *
+     * This is the BusPro-side baseline, not the working edit selection: it answers
+     * "did the participant have this service when the booking was last read back".
+     * Services already held may be kept regardless of their current status or
+     * contingent - withdrawing one produces "Anzahl Leistung stimmt nicht mit
+     * Teilnehmerzuordnung überein" on the next update.
+     *
+     * @param int $participantIndex The participant index to search for
+     * @param int $serviceId        The service ID to look for
+     *
+     * @return bool True if the participant already holds the service
+     */
+    public function hasServiceForParticipant(int $participantIndex, int $serviceId): bool
+    {
+        foreach ([...$this->additionalServices, ...$this->transportationServices] as $service) {
+            if ($service->id === $serviceId && in_array($participantIndex, $service->mapping)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     /**
      * Retrieves transportation service for a specific participant and direction.
      *