[cub] Use CCCL Runtime in merge env/API smoke tests

Part of #2800.

Use CCCL Runtime APIs in a small CUB merge env/API smoke-test batch.

Candidate files:

- `cub/test/catch2_test_device_merge_env_api.cu`
- one adjacent merge env/API file if appropriate

Apply Runtime buffer, stream, copy, or helper patterns where they fit, but do not change CUB API behavior under test. Keep raw CUDA calls when they are the behavior being tested.