@DataJpaTest + Testcontainers exercises the V56 migration plus the
@ElementCollection wiring end-to-end. Saves a block with two PersonMention
entries, clears the persistence context, reloads, asserts both entries
return with their personId + displayName intact. Second test guards the
@Builder.Default — a block without explicit mentions reloads with an empty
list, not null.
Refs #362
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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