Add public SDK cancel endpoint and CloudClient method

- POST /v1/tasks/{task_id}/cancel: tasks:submit scoped, 200 for
  immediate/idempotent cancellation, 202 for newly recorded pending
  cancellation, 404 for unknown task, 409 for terminal task.
- TaskCancellationResponse{task_id, status} model.
- Widen list_tasks status_filter Literal to include "cancelled".
- CloudClient.cancel_task(task_id).
- SDK-level tests covering queued/assigned/idempotent/404/409/scope
  cases for both the router and CloudClient.

Task 5/9 of task-cancellation change.

packages/cloud-platform/cloud/sdk/client.py

@@ -115,6 +115,10 @@ class CloudClient:
         resp = self._request("GET", f"/tasks/{task_id}/attempts")
         return resp.json()
 
+    def cancel_task(self, task_id: str) -> dict[str, Any]:
+        resp = self._request("POST", f"/tasks/{task_id}/cancel")
+        return resp.json()
+
     # ----------------------------------------------------------------- devices
 
     def list_devices(self) -> list[dict[str, Any]]: