[ML] Auto-approve backport PRs so auto-merge can complete

[edsavage]

Summary

Backport PRs opened by the auto-backport workflow have auto-merge enabled but never merge (e.g. #3092). The release branches require one approving review via the org-wide ruleset [org] Require a PR (id 221255, source: elastic), which has no bypass actors. The backport PR was authored by github-actions[bot], which (a) cannot approve its own PR and (b) does not trigger CI on the PR it opens — so the required review is never satisfied and auto-merge stays BLOCKED.

What this PR changes

Adopts the production-proven author-with-ephemeral-token / approve-with-GITHUB_TOKEN pattern (used by elastic/cloud, elastic/elasticsearch-ruby and the ECP GitOps repos such as elastic/catalog-sync-config):

• Mints a short-lived token from Elastic's central ephemeral-token service (elastic/ci-gh-actions/fetch-github-token@v1, ci-prod, via OIDC->Vault) before the backport action and passes it as the action's github_token, so the backport PRs are authored by the ephemeral-token identity. This makes them (a) a distinct identity from github-actions[bot] and (b) subject to CI.
• Approves each backport PR with GITHUB_TOKEN (github-actions[bot]) — a different identity from the author, so the approval counts — then enables auto-merge (squash).
• Adds id-token: write for OIDC.
• Safe no-op before the Token Policy is registered: the token fetch uses continue-on-error and the workflow falls back to authoring with GITHUB_TOKEN; auto-approval is skipped with a warning and the backport awaits a manual approval.

Why this counts (verified, not assumed)

elastic/catalog-sync-config merges renovate PRs under the identical org-only ruleset (221255 + 1225254; count: 1, require_code_owner_review: false, require_last_push_approval: false — same as ml-cpp): a PR authored by one bot and approved by github-actions[bot] (a non-author identity) shows reviewDecision: APPROVED and auto-merges (e.g. elastic/catalog-sync-config#83, elastic/catalog-info#4264). ml-cpp already has Allow GitHub Actions to create and approve pull requests enabled (can_approve_pull_request_reviews: true). GitHub's only relevant restriction is self-approval, which the author != approver split avoids.

Required to activate (not in this repo)

Register the GitHub Token Policy in elastic/catalog-info (elastic/catalog-info#4281), scoped as:

• bound_claims.workflow_ref: elastic/ml-cpp/.github/workflows/backport.yml@*
• permissionset.additional_permissions: { contents: write, pull_requests: write } (contents to push the backport branch, pull_requests to open the PR), selector repository: elastic/ml-cpp
• owner: group:ml-core

Notes

• The earlier approach (approve with the ephemeral token) relied on an app holding only pull_requests: write being an eligible approver, which has no production precedent in the org. This pattern instead approves with GITHUB_TOKEN, which is proven to count.
• elastic/ci-gh-actions/fetch-github-token@v1 is used by tag; happy to pin to a SHA if org policy requires it.

Test plan

• Merge with the Token Policy unregistered -> a subsequent backport still opens the PR (authored by github-actions[bot]) + arms auto-merge, logs the "ephemeral token unavailable" warning, and does not fail.
• Register the Token Policy, trigger a backport, and confirm the PR is authored by the ephemeral-token identity, CI runs, github-actions[bot] approves it, and it merges once checks pass.

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