internal/aws: re-enable k8s shutdown test

[MukundaKatta]

Description

Re-enable TestGetShutdown by closing the temporary kubeconfig file handle immediately after creating it. Leaving the handle open can prevent Windows cleanup from removing the temp file after the test starts Kubernetes informers.

Fixes #43918

Link to tracking issue

Fixes #43918

Testing

• go test ./k8sclient from internal/aws/k8s
• git diff --check

Documentation

No documentation changes required.

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