_merge_assistant_into never joins multimodal (list) content, so a non-empty later turn
beside a list (or a list beside non-empty text) is dropped, yet the fold still recorded its
uid in _absorbed_message_uids, a false claim that its text lives on. The merge now reports
whether the later text was kept and the witness follows it; the retired row id is still
recorded.
Reported by @andrexibiza on #126307.
5d8b17f865