docs: add Helm to CONTRIBUTING prerequisites for chart tests

[SatvikMishra08]

Summary

CONTRIBUTING.md listed Bun/Node and Git, then told contributors to run bun run test. Chart unit tests need Helm 4.1.3 (CI pin) and helm dependency build for the gitignored PostgreSQL subchart. Without them, a fresh clone gets ~166 helm PATH failures (see #569).

Docs-only:

• Add Helm 4.1.3 + helm repo add / helm dependency build … --skip-refresh under Prerequisites
• Note the failure signature next to bun run test, and prefer bun run test:ci for a CI-matching check
• Document the stale docker login → 401 trap

Closes #570

Test plan

• Fresh clone follow Development Setup → Helm install + dependency build → chart unit files runnable
• Skim CONTRIBUTING: Helm appears once in Prerequisites; bun run test note does not duplicate the Helm Chart Changes section

Docs-only change; local bun run test:ci not run (full suite is heavy for a CONTRIBUTING.md edit).

[cevheri]

Thanks, this matches what was measured on a fresh checkout: the twelve files, the 166 errors, the two commands and the 401 trap are all right. Two small changes before merge, both attached as suggestions you can apply with one click.

1. The bun run test line in the scripts block is one long comment and carries two phrases that will not age well: the single-process failures are deterministic rather than a flake, and nothing tracks them yet. How the test script itself gets fixed is a maintainer decision and will be written down where it lands, so keep this line to the Helm fact and let the test:ci line say what it is.
2. The file count in Prerequisites reads as thirteen (the glob plus two names); ten of the eleven helm-chart-* files spawn helm, so name the ten.

[cevheri]

Count and exact number:

Suggested change

- [Helm](https://helm.sh/) 4.1.3 (the version CI runs). Twelve unit files under `tests/unit/` (`helm-chart-*.test.ts`, `distribution-check.test.ts`, `generate-channel-showcase.test.ts`) spawn the `helm` binary. Without it on `PATH`, `bun run test` fails with many `error: Executable not found in $PATH: "helm"` errors. The PostgreSQL subchart tarball is gitignored (`*.tgz`), so a fresh clone also needs:

- [Helm](https://helm.sh/) 4.1.3 (the version CI runs). Twelve unit files under `tests/unit/` (ten `helm-chart-*.test.ts` files, `distribution-check.test.ts` and `generate-channel-showcase.test.ts`) spawn the `helm` binary. Without it on `PATH`, `bun run test` fails with 166 `error: Executable not found in $PATH: "helm"` errors. The PostgreSQL subchart tarball is gitignored (`*.tgz`), so a fresh clone also needs:

[cevheri]

Suggested change

	bun run test # every test layer; never bare `bun test`. Chart tests need Helm + the built subchart (see Prerequisites); without Helm you get ~166 `Executable not found in $PATH: "helm"` failures. Prefer `bun run test:ci` for a CI-matching green run (the interactive `test` script still has a separate single-process flake tracked outside this docs fix).
	bun run test # every test layer; never bare `bun test`. Needs Helm and the built subchart, see Prerequisites

[cevheri]

Suggested change

	bun run test:ci # CI unit/integration lane (green when Prerequisites are met)
	bun run test:ci # the same layers with one process per file, which is what CI runs; use it to verify