Adds _session_message_dedup_key (full-precision timestamp) so true duplicates
(same role + content + EXACT timestamp) fold, while legitimately-repeated
identical turns with sub-second-distinct timestamps survive — avoiding the
#3268 data-loss class. Wired into both the no-sidecar path and the merge loop's
seen_dedup_keys guard. Closes#3346.
Co-authored-by: thanhtoantnt <thanhtoantnt@users.noreply.github.com>
