CI: install-test the example values files

[aktech]

Split out from #127: that issue covers authoring the example files (examples/nebari-values.yaml, examples/standalone-values.yaml if applicable, examples/argocd-application.yaml). This one covers making CI prove they actually work, so the examples cannot rot.

Today CI only lints and templates with the NebariApp disabled and never touches any example values:

https://github.com/nebari-dev/nebari-data-science-pack/blob/bcf0eb7a8c5b07dce58ce3053c2a7ea729e562f3/.github/workflows/lint.yaml#L24-L27

Acceptance criteria

• helm template renders the chart with each file under examples/ (hostname placeholder substituted) and the output passes the manifest schema validation from CI: render chart with NebariApp enabled and add manifest schema validation #124
• A kind-based CI job runs helm install with the example values (standalone example, or the nebari example with nebariapp.enabled=false if pack-metadata.yaml: declare nebariapp_integration, scope, and product_owner #122 lands on standalone-supported: no) and waits for the hub to become Ready
• Core features verified working on that install, not just "pods up": user login, server spawn reaches a running JupyterLab, shared storage mounted and group-writable, jhub-apps landing page serves. Reuse the existing e2e suite against the example-values deployment rather than writing a parallel one
• The ArgoCD Application example is schema-validated as part of the same job
• Job runs on push/PR alongside the existing lint workflow

Related

• Add example values files and an ArgoCD Application example #127 (authoring the examples)
• CI: render chart with NebariApp enabled and add manifest schema validation #124 (kubeconform / schema validation step this builds on)
• pack-metadata.yaml: declare nebariapp_integration, scope, and product_owner #122 (standalone-supported decision gates which example gets the install test)