fix(agentex): publish task_updated when a task is failed via the service

AgentTaskService.fail_task persisted FAILED via task_repository.update without
publishing a task_updated event — the only terminal write that didn't emit.
An actively streaming viewer therefore never learned the task failed and its
SSE stream blocked forever (an eventless terminal transition).

Route fail_task through update_task so it emits like every other terminal
write. Now all terminal transitions publish an event, which the SSE in-loop
check reacts to — no eventless-terminal leak.

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

Author: deepthi-rao-scale

agentex/src/domain/services/task_service.py

@@ -166,7 +166,9 @@ async def forward_task_to_acp(
     async def fail_task(self, task: TaskEntity, reason: str) -> None:
         task.status = TaskStatus.FAILED
         task.status_reason = reason
-        await self.task_repository.update(task)
+        # Publish task_updated so streaming viewers see the failure and end.
+        # Every terminal write must emit; SSE termination relies on it.
+        await self.update_task(task)
 
     async def get_task(
         self,
@@ -374,7 +376,11 @@ async def cancel_task(
             new_status=TaskStatus.CANCELED,
             status_reason="Task canceled by user",
         )
-        return updated if updated is not None else await self.task_repository.get(id=task.id)
+        return (
+            updated
+            if updated is not None
+            else await self.task_repository.get(id=task.id)
+        )
 
     async def interrupt_task(
         self, agent: AgentEntity, task: TaskEntity, acp_url: str