Regression guard for the @NotNull on PersonMention.personId paired with @Valid on the DTO field. The wiring was added in the previous commit; this test ensures dropping either annotation in the future causes a loud test failure rather than silently allowing payloads with no personId to reach the service layer (where the listener relies on the UUID being present). Refs #362 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>