Fix maxTransfers

Author: optionsome

src/main/java/org/opentripplanner/apis/gtfs/mapping/routerequest/TransitPreferencesMapper.java

@@ -75,10 +75,18 @@ static void setTransitPreferences(
         }
         var maxTransfers = transfer.getGraphQLMaximumTransfers();
         if (maxTransfers != null) {
-          transferPreferences.withMaxTransfers(maxTransfers);
+          if (maxTransfers < 0) {
+            throw new IllegalArgumentException("Maximum transfers must be non-negative.");
+          }
+          transferPreferences.withMaxTransfers(maxTransfers + 1);
         }
         var additionalTransfers = transfer.getGraphQLMaximumAdditionalTransfers();
         if (additionalTransfers != null) {
+          if (additionalTransfers < 0) {
+            throw new IllegalArgumentException(
+              "Maximum additional transfers must be non-negative."
+            );
+          }
           transferPreferences.withMaxAdditionalTransfers(additionalTransfers);
         }
       }

src/test/java/org/opentripplanner/apis/gtfs/mapping/routerequest/RouteRequestMapperTransitTest.java

@@ -89,7 +89,7 @@ void testTransferPreferences() {
     assertEquals(cost.toSeconds(), transferPreferences.cost());
     assertEquals(slack.toSeconds(), transferPreferences.slack());
     assertEquals(maximumAdditionalTransfers, transferPreferences.maxAdditionalTransfers());
-    assertEquals(maximumTransfers, transferPreferences.maxTransfers());
+    assertEquals(maximumTransfers + 1, transferPreferences.maxTransfers());
   }
 
   @Test