test: add unit tests for k8s-helper kubernetes API functions

[jeffspahr]

Summary

Adds comprehensive unit tests for kubernetes client functions in k8s-helper.ts to improve test coverage ahead of the planned @kubernetes/client-node upgrade from 0.16.3 to 1.4.0.

Tests added for:

• getPodLogs: success, default container, empty response, API errors
• getPod: success, not found, invalid resource name validation
• getConfigMap: success, not found, invalid resource name validation
• listPodEvents: success, API errors, invalid resource name validation
• getArgoWorkflow: namespace validation
• getK8sSecret: base64 decoding, missing keys, empty data, API errors, namespace validation

Coverage improvement for k8s-helper.ts:

Metric	Before	After
Statements	72.66%	90.64%
Branches	54.23%	84.74%
Functions	68.42%	100%
Lines	71.54%	90.24%

Test plan

• All 33 k8s-helper tests pass
• All 153 server tests pass
• Tests follow existing project patterns (jest.spyOn on TEST_ONLY exports)

[google-oss-prow]

Hi @jeffspahr. Thanks for your PR.

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[google-oss-prow]

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