fix(agentex): end SSE stream when the task row no longer exists

The periodic status recheck's get_task raises ItemDoesNotExist for a
hard-deleted task (e.g. retention cleanup). That was caught by the generic
handler and retried with backoff indefinitely, spinning error events and
holding SSE + DB connections until the client disconnected. Catch
ItemDoesNotExist specifically and end the stream — a vanished row is terminal.
Other exceptions still fall through to the transient retry path.

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

@@ -5,6 +5,7 @@
 from fastapi import Depends
 from pydantic import ValidationError
 
+from src.adapters.crud_store.exceptions import ItemDoesNotExist
 from src.adapters.streams.adapter_redis import DRedisStreamRepository
 from src.api.schemas.task_stream_events import TaskStreamEvent
 from src.config.dependencies import DEnvironmentVariables
@@ -167,7 +168,15 @@ async def stream_task_events(
                     # in case a terminal event's publish was lost.
                     if current_time - last_status_check >= ping_interval:
                         last_status_check = current_time
-                        task = await self.task_service.get_task(id=task_id)
+                        try:
+                            task = await self.task_service.get_task(id=task_id)
+                        except ItemDoesNotExist:
+                            # Row permanently gone (e.g. retention) — end, don't retry.
+                            logger.info(
+                                f"Ending SSE stream for task {task_id}: "
+                                "task no longer exists"
+                            )
+                            return
                         if task.status in TERMINAL_TASK_STATUSES:
                             logger.info(
                                 f"Ending SSE stream for task {task_id}: "