Remove Test-Designer

To clean up the Java DSL, improve the framework’s maintainability and reduce the complexity from a user´s perspective, we are going to remove the TestDesigner.

Start by locating the TestDesigner entry points and references in the Java DSL. Read the surrounding framework code to determine the full removal scope and any affected integration tests. Done means the TestDesigner is removed without leaving broken Java DSL references, with the relevant test suite passing.

Written by the indexing model from the issue text.