add render helm chart validation in abs (detect duplicate keys and general YAML problems)

[piontec]

[weatherhog]

Verified still valid: abs had no helm template invocation at all — ct lint doesn't render templates, and nothing detected duplicate keys (PyYAML and Helm both silently keep the last duplicate).

Implementation: giantswarm/app-build-suite#567 — new HelmTemplateValidator step renders the chart offline and fails the build on duplicate mapping keys or YAML syntax errors in the rendered manifests, pointing at the originating template file and lines. Fail-hard and enabled by default, with --disable-helm-template-validator and --helm-template-extra-values as escape hatches.

Nice validation of the idea: the new step immediately found a real duplicated helm.sh/chart label in abs's own hello-world-app example chart.

[weatherhog]

PR merged. done