test(audit): guard await_args against None for mypy union-attr

CI mypy (which checks tests/, unlike the targeted source-only run that
missed it) flagged ack_mock.await_args.args[1] — await_args is
_Call | None. Assert it is not None first, matching the spawn_call
pattern above.

tests/unit/runtime/test_scheduled_audit_trigger.py

@@ -191,7 +191,9 @@ async def test_reactive_alert_acks_so_it_cannot_rotate(
     assert spawn_mock.await_count == 1  # not respawned on tick 2
     assert ack_mock.await_count == 1
     # acked as the auditor saw it: (client, alert_id)
-    assert ack_mock.await_args.args[1] == alert_id
+    ack_call = ack_mock.await_args
+    assert ack_call is not None
+    assert ack_call.args[1] == alert_id
 
 
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