MockNotFoundEntityMarker: remove reliance on "desc"

[01es]

Description

The current implementation of MockNotFoundEntityMaker relies on the fact that AbstractEntity has property desc: String to capture string values, entered by users. However, property desc will be removed from AbstractEntity at some stage in the future.

A separate, specifically generated property to hold values typed by users should be introduced instead of using desc.

Expected outcome

More future-proof implementation of MockNotFoundEntityMarker.