Rename UnprocessableEntity strings in backend to UnprocessableContent for consistency

[corneliusroemer]

Just to note - although the warnings are fixed, the backend code itself still has plenty (165) of references to 'unprocessable entity': e.g. handleUnprocessableEntityException, UnprocessableEntityException, test titles reference unprocessable entity and unprocessableEntity

Originally posted by @tombch in #7142 (comment)

[claude]

This issue is related to #6976, which addresses the user-facing display of 'Unprocessable Entity' on the website. This issue (#7170) complements that by focusing on backend code consistency and renaming internal references to 'UnprocessableContent'.