fix(initializer): propagate cache initializer Kubernetes API failures

[sahilleth]

Summary

• Re-raise Kubernetes API errors from cache dataset initializer instead of exiting successfully
• Treat AlreadyExists (409) as idempotent for ServiceAccount, LeaderWorkerSet, and Service
• Track resources created in the current invocation and only clean those up on failure (reverse order: Service → LWS → SA)
• Re-raise the original error after best-effort cleanup; cleanup failures do not mask it

Fixes #3835

Test plan

• pytest pkg/initializers/dataset/cache_test.py -v (24 tests)
• pytest pkg/initializers/dataset/main_test.py -v (cache ApiException propagates through main())
• TrainJob GET failure propagates ApiException
• ServiceAccount / LeaderWorkerSet / Service create failures propagate and clean up only owned resources
• All three resource types handle 409 idempotently without deleting pre-existing objects
• Readiness poll GET failure cleans up all newly created resources
• Cleanup delete errors are logged without replacing the original failure

[google-oss-prow]

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