fix(agentex): don't close a live task's stream when its idle key expires

Address review: the fallback vanished-topic check closed the SSE stream for a
still-running task whose sliding-TTL stream key was reclaimed after an idle
period, even though a later XADD would recreate the key. Only treat a vanished
topic as terminal when the task's status could not be confirmed (lookup failed
/ hard delete); a confirmed non-terminal task stays live.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: deepthi-rao-scale

agentex/src/domain/use_cases/streams_use_case.py

@@ -247,27 +247,31 @@ async def _stream_is_finished(
         stream_ever_existed: bool,
     ) -> bool:
         """
-        Fallback terminal check. True if the task is terminal, or the topic
-        was reclaimed after having existed (covers hard-deleted tasks whose
-        lookup fails). A not-yet-written topic is not treated as finished. A
-        transient lookup failure keeps the stream open.
+        Fallback terminal check. True if the task is terminal, or the task's
+        status could not be confirmed AND its topic was reclaimed after having
+        existed (covers a hard-deleted task or a failing lookup). A confirmed
+        non-terminal task is treated as live even if its idle key was
+        reclaimed, and a not-yet-written topic is not treated as finished.
         """
         try:
             task = await self.task_service.get_task(id=task_id)
         except Exception as e:
             logger.warning(
                 f"Terminal-state lookup failed for task {task_id}; "
-                f"keeping stream open: {e}"
+                f"treating status as unknown: {e}"
             )
             task = None
         # Normal finish: the DB says the task is done, so no more XADDs.
         if task is not None and task.status in _TERMINAL_TASK_STATUSES:
             return True
-        # Vanished topic: the key existed before but its sliding TTL reclaimed
-        # it, so a reader is now blocking on a key that can never return data
-        # (also covers a hard-deleted task whose status lookup failed above).
-        if stream_ever_existed and not await self.stream_repository.stream_exists(
-            stream_topic
+        # Vanished topic, but only when the status is unknown (lookup failed /
+        # hard delete). A confirmed non-terminal task is still live even if its
+        # idle key was reclaimed by the sliding TTL — a later XADD recreates the
+        # key, so we must not close its stream on the expired key alone.
+        if (
+            task is None
+            and stream_ever_existed
+            and not await self.stream_repository.stream_exists(stream_topic)
         ):
             return True
         return False