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
2 files changed