[backport v1.6] ci: Add merge queue enforcement workflow (#4335)

* Add merge queue enforcement workflow

Agent-Logs-Url: https://github.com/Azure/azure-container-networking/sessions/ea33035b-0e79-4882-918b-7414023e1e13

Co-authored-by: jpayne3506 <89417863+jpayne3506@users.noreply.github.com>

* ci: correct ai work

* ci: update regex and trigger

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: jpayne3506

.github/workflows/merge-queue-enforcement.yaml

@@ -0,0 +1,47 @@
+name: 'Merge Queue Enforcement'
+on:
+  pull_request:
+    types:
+      - opened
+      - reopened
+      - synchronize
+      - ready_for_review
+    branches:
+      - master
+      - release/v[0-9]*.[0-9]*
+  merge_group:
+    types:
+      - checks_requested
+
+permissions:
+  contents: read
+
+jobs:
+  merge-queue-gate:
+    name: Merge Queue Gate
+    runs-on: ubuntu-latest
+    steps:
+      - name: Verify merge queue rule is active on target branch
+        if: github.event_name == 'pull_request'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          REPO: ${{ github.repository }}
+          BRANCH: ${{ github.base_ref }}
+        run: |
+          if [[ ! "$BRANCH" =~ ^(master|release/v[0-9]+\.[0-9]+)$ ]]; then
+            echo "::error::Unexpected base branch name: '$BRANCH'. Expected 'master' or 'release/vX.Y'."
+            exit 1
+          fi
+          RULES=$(gh api "/repos/$REPO/rules/branches/$BRANCH" \
+            --jq '[.[] | select(.type == "merge_queue")] | length')
+          if [[ "$RULES" -eq 0 ]]; then
+            echo "::error::No active merge queue rule found for branch '$BRANCH'."
+            echo "A branch ruleset with the 'merge_queue' rule must be configured."
+            echo "Until this is fixed, direct merges into '$BRANCH' are possible."
+            exit 1
+          fi
+          echo "Merge queue rule is active on '$BRANCH' – check passed."
+
+      - name: Confirm merge queue context
+        if: github.event_name == 'merge_group'
+        run: echo "Running in merge queue context – check passed."