Adopt helm-unittest for chart template testing

## Problem

All chart testing relies on `ct lint` + `ct install` with CI values files. Each file triggers a full Helm install into a kind cluster, so adding a file per feature permutation compounds CI time.

## Proposal

Adopt [helm-unittest](https://github.com/helm-unittest/helm-unittest) for fast, template-level testing alongside the existing CI setup:

- **helm-unittest**: fast YAML-based tests for value permutations, conditionals, and edge cases (no cluster needed, runs in seconds)
- **ct install**: keep a small set of CI values files for **happy-path integration tests only**

This hybrid approach is used by ingress-nginx and prometheus-community.