chore: Check if pubspec.lock is up to date in CI

.github/workflows/integrate.yaml

@@ -17,6 +17,8 @@ jobs:
           flutter-version: ${{ env.FLUTTER_VERSION }}
           cache: true
       - run: flutter pub get
+      - name: Check if pubspec.lock is up to date
+        run: git diff --exit-code pubspec.lock
       - run: flutter gen-l10n
       - name: Check formatting
         run: dart format lib/ test/ --set-exit-if-changed