feat(ci): add helm-unittest to CI pipeline and mise tasks

[sauagarwa]

Problem Statement

The Helm chart now has unit tests (e.g., deploy/helm/openshell/tests/sandbox_namespace_test.yaml) but there is no CI workflow or mise task to run them. The helm-unittest plugin is not referenced anywhere in the project — no CI workflows, mise tasks, or Makefiles install or invoke it.

Proposed Design

1. Add a mise task (e.g., tasks/helm.toml or extend an existing task file) that:

   • Installs the helm-unittest plugin if not present (helm plugin install https://github.com/helm-unittest/helm-unittest)
   • Runs helm unittest deploy/helm/openshell

2. Add a CI step to the branch-checks or CI workflow that runs the Helm unit tests after chart linting.

Alternatives Considered

• Bundling this into individual Helm PRs — rejected because it's a cross-cutting infrastructure concern that should be set up once, not per-feature.
• Only running tests locally — insufficient; chart regressions would only be caught if contributors remember to run tests manually.

Agent Investigation

Searched the project for any existing helm-unittest references:

• No hits in .github/workflows/, tasks/, mise.toml, mise.lock, or Makefile
• The plugin is not a project dependency — only available if individually installed on a developer's machine
• Chart tests exist at deploy/helm/openshell/tests/ but are never executed in CI

[TaylorMutch]

This would be a great addition!