Add tests for Kubernetes client in the agent

MR: Pending

Description

Followup from: gitlab-org/cluster-integration/gitlab-agent!1811 (comment 2147622571)

The Kubernetes client interface used in the agent by the workspaces domain has been refactored to be more generic, most of these APIs are thin wrappers around a Kubernetes client call. There do exist some functions that would benefit from unit-test coverage within that package. We should write tests for these to make the client wrapper more robust.

Acceptance Criteria

• Write appropriate unit tests for methods in the client where sensible and applicable