chart: add topologySpreadConstraints defaults

[ChongZhe001025]

Summary

Adds missing topologySpreadConstraints defaults to the legacy actions-runner-controller chart values.

The chart templates and README already support topology spread constraints for:

• the controller pod
• the GitHub webhook server pod
• the actions metrics server pod

However, the corresponding default entries were missing from charts/actions-runner-controller/values.yaml, unlike similar scheduling-related fields such as nodeSelector, tolerations, and affinity.

Context

Refs #4097.

While investigating the issue, I found that the templates already render:

• .Values.topologySpreadConstraints
• .Values.githubWebhookServer.topologySpreadConstraints
• .Values.actionsMetricsServer.topologySpreadConstraints

The README also documents these values, so this change only adds the missing defaults to values.yaml for consistency and discoverability.

Changes

• Added root topologySpreadConstraints: []
• Added githubWebhookServer.topologySpreadConstraints: []
• Added actionsMetricsServer.topologySpreadConstraints: []

Testing

• Ran git diff --check
• Ran helm template for the controller topology spread constraints
• Ran helm template for githubWebhookServer.topologySpreadConstraints
• Ran helm template for actionsMetricsServer.topologySpreadConstraints