CI: fetch PR head ref before checkout to prevent test job SHA misses

[Copilot]

test failed in Actions during checkout because GITHUB_SHA was not present in the local mirror after PR sync. The workflow fetched only the PR merge ref, while some runs require the PR head commit object to resolve checkout.

• Root-cause scope

  • Checkout path in CI mirror sync for pull_request runs (.github/workflows/ci.yml) could miss GITHUB_SHA objects.

• Workflow sync hardening

  • In both test and benchmark jobs, retained merge-ref fetch and added head-ref fetch:
    • refs/pull/${PR_NUMBER}/merge
    • refs/pull/${PR_NUMBER}/head
  • No changes to test commands or benchmark logic; only fetch coverage before checkout.

• Representative change

echo "Fetching PR #${PR_NUMBER} merge ref..."
retry "git fetch PR #${PR_NUMBER}" git fetch origin "refs/pull/${PR_NUMBER}/merge" || exit 1
echo "Fetching PR #${PR_NUMBER} head ref..."
retry "git fetch PR #${PR_NUMBER} head" git fetch origin "refs/pull/${PR_NUMBER}/head" || exit 1