test(optimizer): add unit tests for OptimizerClient in KubernetesBackend

This adds comprehensive unit tests for the OptimizerClient methods in
KubernetesBackend, mirroring the existing test structure used for
TrainerClient in backend_test.py.

Tests cover:
- get_runtime (success, timeout, runtime error)
- list_runtimes
- optimize (study creation)
- get_study
- list_studies
- get_study_logs
- delete_study

All tests use pytest fixtures, mock Kubernetes API calls
closes kubeflow#124

Author: vrd-cse