InputStream inputStream = DishDecider.class.getResourceAsStream("/dish-decision.dmn11.xml");
In your example you use the Spring ImportResource annotation which is used to import bean definitions. That is why Spring tries to validate the DMN XML file you specified and correctly fails as it isn't in the correct format. Remove this line.
The XSD is only validated by the Camunda DMN Model API which has the XSD as a resource. The one I linked in a previous comment.
one doubt, I am not good in xml but just curious if I brows xsd url for dmn in browser it does not work. Then how it gets xsd to validate xml?