test(self-managed): add explicit Helm 3 toolchain gate for tests

The self-managed stack tests should fail early when the local toolchain is unsupported.

Today, a developer with Helm 4 first on PATH can hit a noisy Helmfile failure before the test reaches the stack behavior it is meant to validate:

panic: error determining helm version
Error: unknown flag: --client

That failure is technically accurate, but it is poor developer feedback. The stack supports Helm 3, so the test entry point should detect an unsupported Helm major version before invoking Helmfile render logic.

Expected behavior:

Error: Helm version 4.x is not supported.
This stack requires Helm 3.x.
Use Helm 3 or put the repository-provided supported toolchain first on PATH.

Suggested scope:

• Add a non-interactive toolchain check for the test path.
• Verify Helm major version is 3.
• Verify Helmfile is in the supported version range for this stack.
• Verify common test dependencies such as jq and yq are available.
• Keep the failure direct and actionable; tests should not prompt interactively.

Why this matters:

The tests are supposed to catch stack rendering and value-wiring regressions. If they fail first because an unsupported Helm major version is on PATH, the signal is misleading and easy to misdiagnose. A clear test-time toolchain gate improves developer experience and makes failures easier to act on.

Locate the self-managed stack test entry point and the existing Helmfile render invocation, then inspect how toolchain failures are currently surfaced. Exercise the test path with Helm 4 first on PATH and with missing dependencies; done means a non-interactive, actionable gate accepts Helm 3 and the supported Helmfile, jq, and yq versions.