Written by devonh in review of #20435. It failed on the earlier design,
where an initial sync always recorded what it sent. It passes now
because a response records only if it ends after `_newest`, and fails
if that check is removed.
It complements `test_two_clients_on_one_device_do_not_trust_each_other`,
which runs the syncs in the other order. There the incremental sync is
the response that must not record.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXqkiXTFBuzsPRBFRjHUp8