Mock untyped activity - Java SDK

Hi,
We write temporal unit tests in our java springboot application using the @RegisterExtension annotation and instantiating a TestWorkflowExtension.
We have a workflow with multiple activities. However ONE of these is an untyped activity as we wanted this to be executed by another application(another code base and deployment). In the test case how do we mock this untyped activity.

In the workflowImpl we have
private final ActivityStub activity = Workflow.newUntypedActivityStub(TemporalWorkFlowUtil.getActivityOptionByTaskQueue(Constants.TemporalConstants.SOME_QUEUE));

and activity is executed using
activity.execute(“generateExcel”, Void.class, someRequest);

how do i mock the activity and the activity.execute method