Address @antifragileer's review suggestions on #44498:
1. test_partial_write_does_not_mutate_existing_list: pins that a failed
write with mixed valid/invalid items leaves the prior list untouched
(acceptance criterion #3 from #44496).
2. test_merge_new_item_rejects_whitespace_content: verifies whitespace-only
content is rejected for new items in merge mode, guarding against a
future code change that special-cases merge.
2038bd92d2