A streamed preamble ("Let me search...") finalized at a tool boundary
routed through _try_fresh_final, which unconditionally set
_final_response_sent=True even though it is a NON-final segment. The
gateway then reads that flag as "final delivered" and suppresses the
genuine final answer produced on the next API call, so the user silently
gets nothing. Only reproduces with fresh_final_after_seconds > 0.
- _try_fresh_final / _send_or_edit take is_turn_final; the segment-break
call site passes is_turn_final=got_done so only the turn-final answer
marks final-delivered.
- _reset_segment_state clears the final-delivery flags at every tool
boundary as defense-in-depth against any future premature setter.
- Failing-first regression + happy-path no-duplicate test.
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