ci: render Loki Helm chart during validation

[systemdbrew]

Summary

Add a dedicated Helm Render validation job so Loki chart changes are rendered before merge rather than relying only on YAML/Kustomize validation.

The job:

• installs pinned Helm v3.19.0 using a SHA-pinned setup action
• renders the exact Grafana Community Loki 18.13.1 chart with the committed values
• asserts that the rendered output contains the Loki gateway, Longhorn storage class, single-replica replication factor, filesystem object store, TSDB and schema v13

This turns the manual post-merge validation we performed for the Loki migration into a repeatable PR check.

Scope

This intentionally starts with Loki, where external Helm rendering caught the validation gap. Other Argo Helm applications can be added after their values/version compatibility is reviewed rather than creating a brittle generic parser in the first CI change.