Add tests for Kubernetes client in the agent
MR: Pending
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## Description
Followup from: https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/merge_requests/1811#note_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
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