Fix random test failure caused by EntityFile::processStubRow

Problem/Motivation

#2605150: UrlItem::generateSampleValue() does not generate a valid uri introduced a random test failure in EntityFile::processStubRow. The function strips the existing scheme from the the file uri. The scheme may be different in many cases. We replace http:// with public://, this adds two additional chars to the url which trigger the database exception.

The random test failure happened in https://www.drupal.org/pift-ci-job/80178

Proposed resolution

Truncate the new url to max length.

Remaining tasks

User interface changes

API changes

Data model changes