BOLT-1051: Add follow-up tests for VEN registration PR files

Artem asked in Slack thread 1780036222.881059 / msg 1780296139.071089 to make tests for files from BOLT-1051-VenRegistration-Prisma-model-+-migration if any changed files lack tests, and open a new MR/PR. Scope: test-only follow-up against main; inspect original BOLT-1051 PR files, add focused tests for untested files, validate locally, open PR linked to BOLT-1051, monitor CI. Branch TBD artem-agent/bolt-1051-ven-registration-tests.