Fix Use-After-Free in ActionContext destructor. Store temporary task name strings in an explicit std::array to ensure their lifetimes span the entire cancellation loop, avoiding dangling absl::string_view instances when calling CancelCall. Follow-up fix for cl/968097005. PiperOrigin-RevId: 974074293 Change-Id: Idc5534990dedc3fd98aff0184fcf265dd7c15401