feat: implement parity checks for 20ft container bookings to ensure even numbers

apps/edr-freight-api/src/modules/bookings/booking-transition.service.ts

@@ -81,6 +81,28 @@ export class BookingTransitionService {
 
   /** Reject submit when the booking's 20ft containers can't be balanced onto wagons. */
   private async assert20ftPairable(booking: Booking): Promise<void> {
+    // Parity gate. 20ft ride two per wagon, so an odd total leaves one container
+    // that cannot be placed. Consolidation (pairing it with another customer's
+    // odd booking) is built end to end but switched off for now, so an odd total
+    // is rejected here rather than parked for a partner.
+    // containerSize is not always populated (some rows carry only the container
+    // type), so fall back to the type's sizeFt rather than silently skipping
+    // those lines and letting an odd booking through.
+    const ft20Quantity = (booking.bookingContainers ?? [])
+      .filter((bc) =>
+        bc.containerSize
+          ? bc.containerSize.includes("20")
+          : Number(bc.containerType?.sizeFt) === 20,
+      )
+      .reduce((sum, bc) => sum + Number(bc.quantity || 0), 0);
+    if (ft20Quantity % 2 === 1) {
+      throw new BadRequestException(
+        `20ft containers travel two per wagon, so they must be booked in even ` +
+          `numbers. This booking has ${ft20Quantity} — add one more or remove ` +
+          `one (book ${ft20Quantity + 1} or ${ft20Quantity - 1}).`,
+      );
+    }
+
     const violations =
       await this.containerValidationService.validate20ftPairing(booking);
     if (violations.length) {