The manual /compress handler called rewrite_transcript() unconditionally on
the session id returned by _compress_context(). When rotation does not occur
(e.g. _session_db unavailable, or the DB split raised), session_id is unchanged
and rewrite_transcript() DELETEs the original messages and replaces them with
only the compressed summary — permanent data loss (#44794, #39704).
Guard the rewrite on actual rotation: only overwrite when _compress_context
produced a new session id. Otherwise leave the original transcript intact and
log a warning.
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