chore: enable auto-merge on backport PRs (#10108)

## Summary
- Adds `gh pr merge --auto --squash` after backport PR creation in the
backport workflow, so backport PRs merge automatically once checks pass
- Uses `|| echo "::warning::..."` fallback to avoid failing the workflow
if auto-merge can't be enabled (e.g. repo setting not configured)

## Test plan
- [ ] Trigger backport workflow on a test PR with `needs-backport` label
- [ ] Verify auto-merge is enabled on the created backport PR

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10108-chore-enable-auto-merge-on-backport-PRs-3256d73d3650814eb6e5fb2bdf3c5ec7)
by [Unito](https://www.unito.io)

.github/workflows/pr-backport.yaml

@@ -348,6 +348,8 @@ jobs:
               PR_NUM=$(echo "${PR_URL}" | grep -o '[0-9]*$')
 
               if [ -n "${PR_NUM}" ]; then
+                gh pr merge "${PR_NUM}" --auto --squash --repo "${{ github.repository }}" \
+                  || echo "::warning::Failed to enable auto-merge for PR #${PR_NUM}"
                 gh pr comment "${PR_NUMBER}" --body "@${PR_AUTHOR} Successfully backported to #${PR_NUM}"
               fi
             else