KEP-3562: [OptimizationJob] Add End-to-End (E2E) integration tests for OptimizationJob workflow

### What you would like to be added?

Context:
Unit and webhook tests cover static validation and defaulting in [KEP-3562](https://github.com/kubeflow/trainer/pull/3565).

Problem:
We need real cluster tests to verify the interactions between the `OptimizationJob` controller, mutating/validating webhooks, the gRPC Optuna container, and the underlying `TrainJob` instances.

Future Goals:

- Add E2E test suites

- Validate complete job execution cycles: creation -> trial generation -> metric evaluation -> job completion.

- Test failure scenarios (e.g., failing trial recovery, gRPC timeout handling).

Relates to KEP-3562 Master Tracking [Issue](https://github.com/kubeflow/trainer/issues/3562).

### Why is this needed?

E2E tests catch integration bugs in CI/CD pipelines before code is merged, ensuring long-term stability of the OptimizationJob controller.

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