A settled turn that recorded no live duration fell back to
(Date.now()/1000) - session.pending_started_at. But pending_started_at is the
start of an IN-FLIGHT turn, so for a settled turn it is either stale (a session
that sat idle) or about a different pending turn — rendering a bogus
'Processed 15h 32m' on fresh conversations (#4930). Gate the fallback on a turn
actually being in flight (active_stream_id || pending_user_message); otherwise
return undefined (no duration shown) rather than a fabricated one. The recorded
live duration (_turnDuration / base.turn_duration) still takes priority, so this
only removes bogus durations, never a correct one.
