fix: use toYaml with each nodeSelector

[monrax]

Summary

Small fix: render valid YAML for StatefulSet resources configured with node selectors.

What changed

• Use with and toYaml in template fornodeSelector fields in statefulsets/graylog.yaml and statefulsets/datanode.yaml

Linked issues

This closes #69

Checklist

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

Installation

• Fresh installation completes successfully: helm install graylog ./charts/graylog
• All pods reach Running state: kubectl rollout status statefulset/graylog
• Helm tests pass: helm test graylog

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Cluster Configuration
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

Specific to this PR

• Verify node selectors are renderes as valid YAML maps instead of Go map strings (see linked issue)

  helm template graylog ./charts/graylog --set graylog.nodeSelector.kubernetes.os=linux --set datanode.nodeSelector.kubernetes.os=linux | grep "nodeSelector"

Notes for reviewers

• Verify all tests above pass
• Sync up with the author before merging
• The commit history should be preserved - use rebase-merge or standard merge options when applicable