Fix experimental charts and upgrade Helm tooling (#4687)

CONTRIBUTING.md

@@ -101,6 +101,11 @@ NAME=$DOCKER_USER/actions-runner make \
 A set of example pipelines (./acceptance/pipelines) are provided in this repository which you can use to validate your runners are working as expected.
 When raising a PR please run the relevant suites to prove your change hasn't broken anything.
 
+Go chart tests and controller chart-contract tests require the `helm` CLI on `PATH`.
+Install the version listed under [Helm Version Changes](#helm-version-changes)
+before running `go test ./...` or `make test`. The make targets provision envtest,
+not Helm; the helm-unittest plugin is not required for Go tests.
+
 #### Running Ginkgo Tests
 You can run the integration test suite that is written in Ginkgo with:
 
@@ -201,6 +206,13 @@ Send PR, add issue number to description
 In general we ask you not to bump the version in your PR.
 The maintainers will manage releases and publishing new charts.
 
+The Go test job and scale-set chart validation and publishing workflows use
+Helm CLI v4.2.2.
+Keep their `HELM_VERSION` pins aligned when updating the CLI. Validation uses
+helm-unittest v1.1.2 from its signed release archive; the workflow verifies it
+with the pinned upstream signing key. Legacy ARC workflows retain their separate
+Helm version. These CLI pins do not change chart versions or require Helm 4 for users.
+
 ## Testing Controller Built from a Pull Request
 
 We always appreciate your help in testing open pull requests by deploying custom builds of actions-runner-controller onto your own environment, so that we are extra sure we didn't break anything.