[Test][Transform] fix UpdateTransformActionRequestTests failure (#79787)

fix corner case when description gets too long

fixes #79783

Author: Hendrik Muhs

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/transform/action/UpdateTransformActionRequestTests.java

@@ -57,6 +57,10 @@ protected Request mutateInstance(Request instance) throws IOException {
             case 1:
                 String description = update.getDescription() == null ? "" : update.getDescription();
                 description += randomAlphaOfLengthBetween(1, 5);
+                // fix corner case that description gets too long
+                if (description.length() > 1000) {
+                    description = description.substring(description.length() - 1000, description.length());
+                }
                 update = new TransformConfigUpdate(
                     update.getSource(),
                     update.getDestination(),