fix(parser): add default case to the ColumnsTransformer type switch

Satisfies the PMD exhaustive-switch rule (Codacy '1 new issue') and fails
loudly if a future ColumnsTransformerType constant is added without
updating appendTo; unreachable for the current APPLY/EXCEPT/REPLACE
constants, so behavior is unchanged. Same pattern as JsonFunction's
Unhandled JsonOnResponseBehavior guard.

Signed-off-by: 付典 <fudianchn@gmail.com>

Author: fudianchn

src/main/java/net/sf/jsqlparser/expression/ColumnsTransformer.java

@@ -84,6 +84,8 @@ public StringBuilder appendTo(StringBuilder builder) {
             case REPLACE:
                 builder.append("REPLACE(").append(Select.getStringList(replaceItems)).append(")");
                 break;
+            default:
+                throw new IllegalStateException("Unhandled ColumnsTransformerType: " + type);
         }
         return builder;
     }