add notes/remark and refactor round-trip

app-modules/payment/src/Exceptions/PaymentInitiationNotAllowedException.php

@@ -16,6 +16,18 @@ class PaymentInitiationNotAllowedException extends RuntimeException
         );
     }
 
+    /**
+     * A round trip's payment is combined on the outbound leg — the return
+     * leg is marked paid when the outbound leg's payment succeeds
+     * (MarkBookingPaid), never via its own Payment (domain.md §2b).
+     */
+    public static function isReturnLeg(Booking $booking): self
+    {
+        return new self(
+            "Booking [{$booking->booking_ref}] is a round trip's return leg — initiate payment on its linked outbound booking instead."
+        );
+    }
+
     public function render(Request $request): ?JsonResponse
     {
         if ($request->expectsJson()) {