feat: add reusable verify-boilerplate composite action

[TrueFurina]

Summary

Adds a reusable composite action (.github/actions/verify-boilerplate) that wraps this repo's hack/boilerplate/boilerplate.py, so Kubeflow repos can call it directly instead of vendoring and maintaining a divergent copy of the script (which currently drifts and makes the "downloaded from kubeflow/testing @ " comment unverifiable).

Requirements met

• Composite action wrapping hack/boilerplate/boilerplate.py, script sourced from this repo — callers never vendor it
• base-ref input defaulting to github.base_ref (PR base branch), so only changed/new files drive new-file detection
• boilerplate-dir input for repos that keep custom templates (defaults to this repo's templates)
• Sets up Python itself (actions/setup-python) so callers don't each add a setup step
• Non-zero exit with the existing remediation output on failure (boilerplate.py exits 1 with remediation text)

Changes

• New file: .github/actions/verify-boilerplate/action.yml

Verification

• YAML structure follows the composite-action spec (runs.using: composite)
• Defaults reference github.base_ref and github.action_path correctly

[google-oss-prow]

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[juliusvonkohout]

Please use a spell checker.

Repo -> repository

[juliusvonkohout]

Ref -> reference

[juliusvonkohout]

Base-reference

[juliusvonkohout]

Dir -> directory

[jaiakash]

Hi @TrueFurina for the issue, we have PR for it. #1070

Please check that, you can help us by reviewing it.

[tariq-hasan]

Closing this in favor of #1070.

[tariq-hasan]

/close

[google-oss-prow]

@tariq-hasan: Closed this PR.

Details

In response to this:

/close

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