fix(streaming): use truthy-check for _pending_started_at fallback

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>

api/streaming.py

@@ -2054,8 +2054,10 @@ def _run_agent_streaming(
                 agent.ephemeral_system_prompt = _personality_prompt
             _pending_started_at = getattr(s, 'pending_started_at', None)
             # Normal chat-start sets pending_started_at before spawning this thread;
-            # fallback to now only for recovered/legacy flows where that marker is absent.
-            _turn_started_at = _pending_started_at if _pending_started_at is not None else time.time()
+            # fallback to now only for recovered/legacy flows where that marker is absent
+            # or has been zeroed out (e.g. via a buggy migration / manual file edit).
+            # Truthy-check covers None, missing-attr, and 0 uniformly.
+            _turn_started_at = _pending_started_at if _pending_started_at else time.time()
             _previous_messages = list(s.messages or [])
             _previous_context_messages = list(_session_context_messages(s))
             _pre_compression_count = getattr(

tests/test_turn_duration_display.py

@@ -21,10 +21,6 @@ def test_streaming_done_payload_includes_backend_turn_duration():
         "Turn duration should be measured from the persisted pending_started_at "
         "start time, not only from browser-local state."
     )
-    assert "if _pending_started_at is not None else time.time()" in STREAMING_PY, (
-        "The fallback should preserve explicit timestamp values and only use now "
-        "when pending_started_at is absent."
-    )
     assert "recovered/legacy flows" in STREAMING_PY, (
         "The missing-start fallback should be documented so it is not mistaken "
         "for the primary timing path."