Record that a restored summarization event was discarded.
Dropping the event also drops its summary_message, so the next request
re-sends the whole untrimmed history. That is a large, silent token and
latency cost whose only symptom is a slow, expensive turn, and the causes
worth chasing — a checkpoint written by another schema, a partial write, a
cutoff recorded against a different message list — all look identical from the
outside. Log it so a repeat is diagnosable.
Shared with the client rather than kept in the middleware: the client is the side that reads possibly-malformed remote snapshot dicts, so it is the more likely place to meet one, and a discard that is loud on one side and silent on the other is worse than either.
