Backfill model-context turns into the visible transcript before saving the
latest reply while keeping hidden [context compaction] markers out of the
visible transcript, so sidebar/header message counts no longer collapse a
compacted gateway session to a 2-message conversation. Resolves the
transcript-loss regression the v0.51.197 Codex gate flagged on the original.
Co-authored-by: AJV20 <24819659+AJV20@users.noreply.github.com>