Trigger a downstream multi-project GitLab CI pipeline and block on its result using strategy depend

Steps

1. In the upstream .gitlab-ci.yml, add a trigger job: `trigger: project: <GROUP>/<DOWNSTREAM_PROJECT>` (optionally add `branch: <REF>` to target a specific branch or tag).
2. Add `strategy: mirror` (GitLab's current recommendation) or the older `strategy: depend` under `trigger:` so the trigger job blocks until the downstream pipeline finishes and reflects its final status, instead of passing as soon as the downstream pipeline is created.
3. Confirm the user or token triggering the upstream pipeline can start pipelines in the downstream project, and add the downstream project to the upstream project's job token scope allowlist if using CI_JOB_TOKEN.
4. To pull artifacts back from the downstream pipeline in a later job, use `needs: - project: <GROUP>/<DOWNSTREAM_PROJECT> job: <JOB_NAME> ref: <REF> artifacts: true`.
5. Gate downstream-only job behavior with `rules: - if: $CI_PIPELINE_SOURCE == "pipeline"`, since multi-project pipelines set CI_PIPELINE_SOURCE to "pipeline" (parent-child pipelines use "parent_pipeline" instead).