Make validation error sealed type. (#66)

Author: naga-zoom

src/main/java/us/zoom/data/dfence/policies/pattern/models/ValidationError.java

@@ -1,6 +1,6 @@
 package us.zoom.data.dfence.policies.pattern.models;
 
-public interface ValidationError {
+public sealed interface ValidationError {
   String message();
 
   record InvalidContainerPolicyPattern(String message) implements ValidationError {}