_session_turns accumulated every turn's text for the whole session, reset
only at session boundaries, so a never-ending session grew without bound
independent of context compaction.
In append mode each retain ships only the delta since the last watermark
(_session_turns[_last_retained_turn_count:]), and a retained turn is never
read again — sync_turn always slices from the watermark and flush-on-switch
flushes what's left. So after an append retain the buffer drops the retained
prefix (clear() + reset the watermark), bounding it to the un-retained tail.
Overwrite mode is deliberately untouched: legacy/overwrite APIs resend the
whole session each retain because each retain replaces the document, so that
path must keep every turn.
Tests: append trims the retained prefix while shipping every turn exactly
once (no loss, no duplication), stays bounded across 100+ turns, and
overwrite mode keeps the full buffer.
(cherry picked from commit cb77e006754dd4ad5c880f22310d3ac7b3472739)
6e1de4850e