fix: base64-encode runnerGroupSpec to prevent errors in helm.StringPathValuesApplier

[xinWeiWei24]

When invoking helm.StringPathValuesApplier during a PATCH operation, passing runnerGroupSpec as a raw string fails if the body contains special characters "[" or "]", like this:

- patch:
    version: v1
    resource: pods
    namespace: kperf
    patchType: json
    name: test-pod4
    body: |
      [
        {
          "op": "replace",
          "path": "/metadata/labels",
          "value": {}
        }
      ]
  shares: 100

To resolve this:

• runnerGroupSpec is now base64-encoded before being passed to Helm
• Decoded in the Helm template using b64dec before applying toYaml