PR #719 review raised a claim that these three fields fall through to
the "keep unchanged" catch-all in _merge_vcard_properties. The existing
elif branches for NICKNAME/BDAY/CATEGORIES already prevent that, but
the behaviour wasn't pinned by a test. Add a focused TestMergeVcardProperties
class that calls the merge helper directly and asserts:
- Existing NICKNAME/BDAY/CATEGORIES lines are overwritten by new values.
- When the existing vCard has none of these lines, update adds them.
- A URL update doesn't clobber unrelated ORG/NOTE/TEL properties.
If the primary update path ever regresses for these fields, these tests
will catch it immediately.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
