feat(scripts): add bump-sdk.mjs

[mvanhorn]

Summary

Closes #2113. Add scripts/v4/bump-sdk.mjs to automate sdkexamples/ version bumps. The script auto-detects the latest stable Helm v4 release (or accepts an explicit --version), runs go get helm.sh/helm/v4@<version> + go mod tidy + go build ./... in sdkexamples/, and bails non-zero with a clear message when the build fails (e.g. an API rename between releases).

Why this matters

The motivation in #2113 (and the linked review on #2067) was:

• sdkexamples/ needs to track each new Helm v4 release so the docs-site examples stay buildable.
• scripts/v4/bump-version.mjs already automates the docs-site side of a release bump, but it is a Node.js script that intentionally does not shell out to the Go toolchain. Extending it would muddy that contract and pull Go into a Node-only file.
• A standalone script (bump-sdk.mjs) keeps concerns separated and matches the existing per-script directory convention (scripts/v4/).
• Pure version automation cannot solve every release (v4.1.x renamed InsecureSkipTLSverify -> InsecureSkipTLSVerify, which broke fix(sdkexamples): bump helm.sh/helm/v4 to v4.2.0 and fix build #2067 and required a manual code fix). The script detects the go build failure and exits with a message that names that exact failure mode, so the maintainer knows to fix the example code by hand.

Changes

• scripts/v4/bump-sdk.mjs (new, +151 lines). Style and arg parsing match the existing bump-version.mjs (--version, --dry-run, --help).
• No changes to sdkexamples/go.mod or go.sum in this PR. The script is a tool to drive the bump, run separately when the maintainer decides to cut a new docs release.

How to test

Locally, on a clean checkout of main (current SDK pin: v4.1.4, latest stable: v4.2.0):

$ node scripts/v4/bump-sdk.mjs --help
Usage: node scripts/v4/bump-sdk.mjs [options]
...

$ node scripts/v4/bump-sdk.mjs --dry-run
Fetching latest Helm version from GitHub... v4.2.0

=================================================
  Helm SDK examples version bump
  Current  : v4.1.4
  Target   : v4.2.0
  Dir      : sdkexamples/
  DRY RUN  : no files will be written
=================================================

Commands that would run:
  (cd sdkexamples && go get helm.sh/helm/v4@v4.2.0)
  (cd sdkexamples && go mod tidy)
  (cd sdkexamples && go build ./...)

The --version v4.X.Y form was syntax-checked but not run end-to-end against the live registry; that is the next step for the human running the script on the day a release lands.

Acceptance criteria from #2113:

• Script updates sdkexamples/go.mod and go.sum for the specified (or latest) Helm v4 release - via go get + go mod tidy.
• Script runs go build ./... and exits non-zero with a clear error if the build fails - runGo(["build", "./..."], ...) exits with a message naming the API-rename failure mode.
• Supports --version vX.Y.Z flag for explicit version.
• Supports --dry-run to preview without writing.

Fixes #2113

AI-assisted.

[benoittgt]

I'm wondering if it should be called from scripts/v4/bump-version.mjs even behind a flag --bump-sdk, or at least mentionned at the end of the command output.