Finish the enemy engagement cleanup

internal/modules/dnd/validate/enemyevents/engagements/validator_test.go

@@ -2,6 +2,7 @@ package engagements
 
 import (
 	"context"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -35,11 +36,17 @@ func TestValidatorEnforcesOneEngagementPerSubject(t *testing.T) {
 			}
 		})
 	}
+	validator := New(Options{})
 	for _, unitID := range []int{1, 2} {
-		result, err := New(Options{}).Validate(context.Background(), contracts.TypedValidationRequest[dnd.EnemyEventList]{Value: events(event("Ashfang", dnd.EnemyEventKindEngaged, unitID))})
+		value := events(event("Ashfang", dnd.EnemyEventKindEngaged, unitID))
+		before := events(event("Ashfang", dnd.EnemyEventKindEngaged, unitID))
+		result, err := validator.Validate(context.Background(), contracts.TypedValidationRequest[dnd.EnemyEventList]{Value: value})
 		if err != nil || !result.Approved {
 			t.Fatalf("separate scene validation = %#v, %v; want approved", result, err)
 		}
+		if !reflect.DeepEqual(value, before) {
+			t.Fatalf("Validate() mutated separate scene value: got %#v, want %#v", value, before)
+		}
 	}
 }