Add Helm values schema validation for NAuth

[choufraise]

NAuth’s Helm chart currently has no values.schema.json.

This means invalid values, wrong value types, and simple typos in chart configuration are only caught late, if they are caught at all. A values schema would give users earlier feedback during normal Helm workflows such as linting, templating, installing, and upgrading.

We should investigate and add schema validation for the NAuth chart values while keeping the chart value API easy to maintain.