Address additional code review nitpicks

- Fix INamedModelInstance Javadoc: null returned when no JSON key configured
- Add null-guard to WrappedException constructor for fail-fast behavior
- Prevent duplicate suppressed exceptions on repeated unwraps

Author: david-waltermire

core/src/main/java/gov/nist/secauto/metaschema/core/model/INamedModelInstance.java

@@ -65,7 +65,7 @@ default boolean hasJsonKey() {
   /**
    * Get the JSON key flag instance for this model instance, if one is configured.
    *
-   * @return the JSON key flag instance or {@code null} if a JSON key is
+   * @return the JSON key flag instance or {@code null} if no JSON key is
    *         configured
    */
   @Nullable

core/src/main/java/gov/nist/secauto/metaschema/core/util/ExceptionUtils.java

@@ -5,6 +5,7 @@
 
 package gov.nist.secauto.metaschema.core.util;
 
+import java.util.Arrays;
 import java.util.Objects;
 
 import edu.umd.cs.findbugs.annotations.NonNull;
@@ -97,7 +98,7 @@ public static final class WrappedException
      *          the exception to wrap
      */
     public WrappedException(@NonNull Throwable cause) {
-      super(cause);
+      super(Objects.requireNonNull(cause, "cause"));
     }
 
     @Override
@@ -131,7 +132,12 @@ public <E extends Throwable> E unwrap(@NonNull Class<E> wrappedExceptionClass) {
       Throwable cause = unwrap();
       if (wrappedExceptionClass.isInstance(cause)) {
         E unwrappedEx = wrappedExceptionClass.cast(cause);
-        unwrappedEx.addSuppressed(this);
+        // Avoid adding duplicate suppressed exceptions on repeated unwraps
+        boolean alreadySuppressed = Arrays.stream(unwrappedEx.getSuppressed())
+            .anyMatch(s -> s == this);
+        if (!alreadySuppressed) {
+          unwrappedEx.addSuppressed(this);
+        }
         return unwrappedEx;
       }
       throw new IllegalArgumentException(