[ML] Publish ml-cpp-ci rollup commit status for PR builds

[edsavage]

Summary

Part 1 of making auto-backport-and-merge (#3099) safe: publish a single, stable rollup CI status so merges can be gated on it.

Today ml-cpp-pr-builds runs with set_commit_status: false, so the only per-commit signals are the individual Buildkite step statuses ("Build and test on … RelWithDebInfo", etc.). None are marked required, so gh pr merge --auto (armed by the backport workflow) merges as soon as CLA/snyk/clang-format/review are green — before the build/test matrix finishes. That's how the #3080/#3081 backports (#3106–#3111) auto-merged mid-build.

This flips set_commit_status: true (the context ml-cpp-ci is already defined) so buildkite-pr-bot publishes one overall ml-cpp-ci status per PR head commit.

Why a rollup rather than requiring the per-step contexts

• One stable context across main and every release branch (the per-step names differ between pipelines).
• Survives pipeline step renames.
• Robust to partial/label-scoped builds (ci:skip-es-tests, single-platform buildkite build on linux, …) that don't post every per-step context — a required per-step check that never arrives would block the PR forever.

Follow-up (not in this PR)

Once merged and main serves the new config, the next PR build will carry an ml-cpp-ci status. After confirming it transitions pending → success/failure, add ml-cpp-ci to the required status checks on main/9.5/9.4/8.19 so auto-merge waits for CI.

Notes

• No production code touched. This PR only edits .buildkite/pull-requests.json, which is in skip_ci_on_only_changed, so the C++ matrix is intentionally skipped here.

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