Switch the per-turn duration fallback from `is not None` to a truthy check so
None, missing-attr, and an explicit 0 all uniformly fall back to time.time().
Without this, a 0 timestamp (e.g. via a buggy migration or manual file edit)
would yield `time.time() - 0` ≈ wall-clock-since-epoch, displaying nonsense
like 'Done in 56 years 4 months ...'. In practice pending_started_at is always
set via int(time.time()) so this is a hardening fix, not a live-bug fix.
Also drop the brittle source-string assertion in the regression test that
pinned the literal expression. The behavioural test
test_done_handler_persists_duration_on_last_assistant_message already proves
the duration field is set; pinning the source line broke twice during the
v0.50.290 release pipeline alone (Opus tightening + maintainer revert).
Fixes#1595
Signed-off-by: Sanjay Santhanam <51058514+Sanjays2402@users.noreply.github.com>
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