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

[vrd-cse]

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 #126

What this PR does / why we need it:
Adds unit tests for OptimizerClient in the Kubernetes backend, exactly matching the style and structure of the existing TrainerClient tests in backend_test.py.

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
it closes #126

Fixes #126

Checklist:
-[x] Tests added ('tests_OptimizerClient.py`)

• Follows existing test pattern (fixtures, mocks, TestCase)
• All tests pass locally:

  python -m pytest -v kubeflow/trainer/backends/kubernetes/tests_OptimizerClient.py
  

• Docs included if any changes are user facing

[google-oss-prow]

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[andreyvelich]

Thanks for this @vrd-cse!
Please refactor your tests by following tests in Kubeflow Trainer: https://github.com/kubeflow/sdk/blob/main/kubeflow/trainer/backends/kubernetes/backend_test.py

You should have similar tests but for Optimizer: https://github.com/kubeflow/sdk/blob/main/kubeflow/optimizer/backends/kubernetes/backend.py