Refactor DGXCloudExecutor to use client_id and client_secret in tests

- Updated test cases to replace app_id and app_secret with client_id and client_secret for consistency.
- Adjusted mock requests in tests to align with the new authentication parameters.

This change enhances clarity and aligns with the recent updates in the DGXCloudExecutor class.

Signed-off-by: Rakesh Paul <rapaul@nvidia.com>

Author: rapaul-nv