CI: "Validate changelog diffs" job fails on every fork-sourced pull request #10579

Bug

"Lint, build, and test / Validate changelog diffs" fails on every fork-sourced pull request, independent of the PR's content.

Root cause

The validate-changelog-diffs job (.github/workflows/lint-build-test.yml) runs the composite action .github/actions/check-merge-queue-changelogs, whose "Check changelog changes" step computes:

TARGET_REF=$(git merge-base "origin/$BASE_REF" "origin/$PR_BRANCH")

For a PR whose head lives on a fork, origin/$PR_BRANCH does not exist in the job's checkout (fork head branches are fetched as refs/pull/N/merge, not as remote branches under origin), so the job dies before validating anything:

fatal: Not a valid object name origin/feature/use-memory-see-this-sp0
##[error]Process completed with exit code 128.

The job has no skip condition beyond the event type, so this affects every fork-sourced PR.

Evidence

• PR test(assets-controller): add stale-balance scenario integration tests for accounts API v6 #10535 (fork branch feature/use-memory-see-this-sp0) — job run: https://github.com/MetaMask/core/actions/runs/36560718378/job/109380743283
  • The job failed on every commit the PR has ever had — including its very first commit ca6a523772 — i.e., the failure is independent of the PR's changes and occurs before any changelog diff is computed (the per-package Validate changelog (24.x, @metamask/assets-controller) job passes for the same commits).
• PR feat(assets-controller): add transient per-account loading state for user-visible fetches #10229 (fork branch feat/assets-transient-loading-state) — the same job shows FAILURE on its head commits (db682a870f, 47b7803dfd).

Impact

• Every fork-sourced PR shows a red "Validate changelog diffs" check (and the "All jobs pass" aggregate) for a reason unrelated to the PR, making genuinely failing checks harder to spot and fork PRs appear blocked.

Suggested fix direction

In the "Check changelog changes" step, when origin/$PR_BRANCH is unavailable (fork PRs), fetch the PR head ref before computing the merge base, e.g.:

git fetch origin "+refs/pull/${PR_NUMBER}/head:refs/remotes/origin/${PR_BRANCH}"

(or resolve the merge base against refs/pull/${PR_NUMBER}/merge directly, which the checkout already has).

Note: for merge_group events the current logic works, because merge-queue branches do exist under origin.