ci: trigger Helm test workflow only on merge queue

[slolatte]

Summary

• Renames FEATURE_BRANCH_HELM_TEST.yaml → MERGE_QUEUE_HELM_TEST.yaml
• Changes trigger from pull_request to merge_group so the workflow only runs once a PR is added to the merge queue
• Removes the fork-guard job (forks can't be added to the merge queue, the guard is unreachable)
• Removes the if: event_name conditions on build-image (no longer needed)
• Extracts the PR number from github.event.merge_group.head_ref so produced images keep their -pr<N>- tag

Branch-protection coordination

For this workflow to gate merges, branch protection on main must:

1. Have merge queue enabled
2. Reference the new check name Merge queue Helm test / Build and Publish Connectors Docker Image (renamed from Feature branch Helm test / Build and Publish Connectors Docker Image)

Test plan

• Confirm merge queue is enabled on main
• Update required-check names in branch protection if needed
• Add a small no-op PR to the merge queue and verify the workflow runs + the Docker image tag remains <version>-pr<N>-run<id>-a<attempt>

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the Helm test GitHub Actions workflow to run only when a PR enters the merge queue, aligning execution with merge-queue gating rather than running on every pull request update.

Changes:

• Renames the workflow to “Merge queue Helm test” and switches the trigger from pull_request to merge_group.
• Removes the external-fork guard and related if: conditions that were specific to pull_request execution.
• Derives the PR number from github.event.merge_group.head_ref to preserve -pr<N>- image tagging.

Comments suppressed due to low confidence (1)

.github/workflows/MERGE_QUEUE_HELM_TEST.yaml:6

• merge_group is configured without a branches filter, so this workflow will run for merge-queue events on any branch that has merge queue enabled (now or in the future). If this job is intended to gate only main (as implied by the PR description and other workflows), add branches: [ main ] under merge_group: to avoid unexpected runs on release/stable branches.