Sara's QA concerns:
1. PersonControllerTest.updatePerson_returns200_whenGenerationNull was
asymmetric — only checked status 200, no body assertion. Now also
asserts `$.generation` is null in the JSON response, mirroring the
in-range test's body check.
2. New full-stack PUT→DB→GET round-trip in PersonServiceIntegrationTest
(updatePerson_clearGenerationToNull_readsBackNullFromDb) seeds a
person with generation=3, calls updatePerson with generation=null,
flushes the persistence context, and asserts the column reads back
null from the DB. Without this we only had the mocked WebMvcTest
boundary; nothing proved JPA actually wrote SQL NULL.
3. Sibling test (updatePerson_setGenerationToZero_readsBackZeroFromDb)
pins the G 0 end-to-end so a primitive zero can't silently coerce
to null anywhere along controller → service → JPA.
Refs #689
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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