Restore 'Validate pull request' step in validate-promotion job

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/8f63396e-12ed-4827-aa8c-5e4b9661cee9

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

.github/workflows/promote-branch.yml

@@ -186,6 +186,12 @@ jobs:
           echo "Current branch is '${{ env.current-branch }}'"
           exit 1
 
+      - name: 'Validate pull request'
+        if: ${{ env.pull-request-exists == 'true' }}
+        run: |
+          echo "Pull request exists."
+          exit 1
+
   promote-branch:
     name: 'Promote branch ${{ github.ref_name }} to ${{ needs.workflow-variables.outputs.target-branch }}'
     needs: [ workflow-variables, validate-promotion ]