[v3-3-test] Catch lang-SDK Go example module drift before it reaches main (#70568)

[github-actions]

• Catch lang-SDK Go example module drift before it reaches main

kubernetes-tests/lang_sdk/go_example is a separate Go module that resolves
the SDK through a replace onto ../../../go-sdk, so it carries its own copy
of the SDK's indirect requirements. Nothing re-tidies it when a dependency
moves inside /go-sdk, and Dependabot bumps exactly one module per PR.

The blast radius is what makes this worth guarding. Go refuses to build an
inconsistent module, so once such a bump merges, "Kubernetes tests / K8S
Lang-SDK" fails at the Build Go bundle step on every pull request until
someone notices and tidies the module by hand — not just on the PR that
caused it.

Dependabot security updates do not consult .github/dependabot.yml, so no
per-directory configuration prevents this, and a second Dependabot PR for the
example module would merge at a different time and leave main red in between.
The drift has to fail the bump PR itself.

The check is go mod tidy -diff in the example module: it asks exactly the
question the failing CI step asks, never writes to the working tree, and
exits non-zero when the module is untidy.

• Let prek provide the Go toolchain for the tidy check

Static checks run on a runner whose preinstalled toolchains are deleted to
free disk space before prek starts, so the check could never find go there
and failed on every run. Asking prek for the toolchain is how the Go SDK's
own tidy hook already gets one, and it pins the same version everywhere.
(cherry picked from commit bce20ff)

Co-authored-by: Jarek Potiuk jarek@potiuk.com