The previous fix made retain_recent_legal_suffix return the actual dropped
message list, but already_consolidated was still computed with
min(before_last_consolidated, len(dropped)), which assumes dropped messages
are always a prefix. In the else branch (tail has no user messages), dropped
may include messages from after the consolidated prefix, causing
already_consolidated to skip too many and leaving tail messages neither
retained nor raw-archived.
Fix by having retain_recent_legal_suffix return (dropped,
already_consolidated_count) where already_consolidated_count is computed
against original message indices. Also fix last_consolidated update to count
how many retained messages were inside the old consolidated prefix.
baffd6ef92