Backport PRs from github-actions[bot] don't trigger ml-cpp-pr-builds Buildkite CI

[valeriy42]

Summary

Backport PRs opened by github-actions[bot] do not trigger the ml-cpp-pr-builds Buildkite pipeline. As a result the build/test matrix never runs, required checks never report, and the auto-backport auto-merge stalls indefinitely.

Recent examples: #3092 (9.4) and #3093 (9.5), both backports of #3082.

Evidence

Source PR #3082 (author valeriy42, org member with write) received the full Buildkite matrix as StatusContext commit statuses, e.g.:

• Build on Linux/MacOS/Windows … RelWithDebInfo
• Test on …
• Java YAML REST / Multi-Node / Inference Integration Tests
• all linking to buildkite.com/elastic/ml-cpp-pr-builds/builds/2821

Backport PRs #3092 / #3093 (author github-actions[bot]) received none of these. Their only checks are CLA, Check labels, license/snyk, security/snyk, and the Backport GitHub Actions workflow.

This is not a label or path skip:

• PR labels (>non-issue, :ml, backport, auto-backport, vX.Y.Z) match none of skip_ci_labels (skip-ci, jenkins-ci, >test-mute, >docs).
• The changed file is .cc, so skip_ci_on_only_changed does not apply.

Root cause

.buildkite/pull-requests.json gates auto-triggering by author:

"allow_org_users": true,
"allowed_repo_permissions": ["admin", "write"],
"build_on_commit": true,
"build_on_comment": true,

github-actions[bot] is neither an org member nor a repo write/admin, so the Buildkite GitHub integration does not honor build_on_commit for bot-authored backport PRs. Builds can currently only be started via a trigger comment (build_on_comment: true) from an authorized user.

Impact

• Every automated backport PR stalls: auto-backport enables auto-merge "when CI passes", but the required Buildkite checks never start.
• Manual intervention (an authorized user commenting buildkite build this) is required on each backport PR. This defeats the purpose of automated backports and is the motivation behind draft [ML] Auto-approve backport PRs so auto-merge can complete #3094.

Workaround (per PR)

An org member with repo write/admin comments a trigger phrase (matches always_trigger_comment_regex):

buildkite build this

Proposed fix

Allow the backport bot to trigger ml-cpp-pr-builds automatically. Options:

1. Extend the Buildkite PR trigger config to allowlist github-actions[bot] (or add a branch rule for backport/*) so build_on_commit fires for bot-authored backports.
2. Have the backport automation post the trigger comment after opening the PR (using a token whose commenter has write access), so build_on_comment fires.

Coordinate with #3094 ("Auto-approve backport PRs so auto-merge can complete"), which addresses the approval half of the same stall.