Add Buildkite CI pipeline for tests

[kgeller]

Summary

Add a Buildkite pipeline to run make check yamllint (the same command as the
existing GHA test.yml workflow), provisioned via Terrazzo from catalog-info.yaml.

The existing GitHub Actions test workflow is intentionally kept running in parallel
until Buildkite is confirmed green, then will be removed in a follow-up PR.

What's included

• .buildkite/pipeline.yml — single test step on pipelib:0.27.0
• .buildkite/scripts/run-tests.sh — wrapper that runs make check yamllint
• catalog-info.yaml — registers the ecs Backstage Component and buildkite-pipeline-ecs Resource

What happens after merge

1. Terrazzo provisions the ecs pipeline in Buildkite (minutes to ~30 min).
2. The merge commit triggers the first BK build on main.
3. Once green, add buildkite/ecs as a required status check on main.
4. Follow-up PR removes .github/workflows/test.yml and drops the old Unit Tests required check.

Pipeline config choices

• branch_configuration: "main 8.* 9.* 10.*" — covers all active release branches; excludes frozen 1.* branches.
• skip_intermediate_builds / cancel_intermediate_builds — enabled on topic branches only; long-lived branches keep every build.
• No filter_condition yet — deferred until buildkite-pr-bot is wired up (separate PR to elastic/infra + webhook config needed first).
• No .buildkite/pull-requests.json yet — same reason; comment triggers (/test) and changeset-based skips arrive in a follow-up.

[github-actions]

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)