Reading more into the WS.containsString() method, I see it wants the first parameter to be a “ResponseObject”, not a TestObject. Are you familiar with creating a new ResponseObject from a String or casting a TestObject to a ResponseObject (don’t know if we can)?
https://docs.katalon.com/javadoc/com/kms/katalon/core/testobject/ResponseObject.html
If not, then how about try the Groovy method contains (you may have to also you the split method to create an array of strings).