fix: preserve task stop intent and guard remote execution boundaries

backend/services/background_tasks/service.py

@@ -101,10 +101,17 @@ class TaskService:
         task_id = message.payload["task_id"]
         self._jobs[message.job_id] = task_id
         record = await self.get(task_id)
+        if record.status == "cancel_requested":
+            await self._stop_record(record)
+            return {"task_id": task_id}
         executor = self.executor_factory(record.connection, record.secrets)
         resume = bool(record.remote_handle)
         try:
             if not resume:
+                outstanding = await self.repository.list(active=True)
+                if any(other.id != task_id and other.remote_handle for other in outstanding):
+                    await self.repository.patch(task_id, status="failed", summary="另一远端任务尚未结束，请先核查或停止该任务；本任务未提交到远端")
+                    return {"task_id": task_id}
                 handle = await executor.prepare()
                 record = await self.repository.patch(task_id, remote_handle=handle)
             last_event = None