ci: add a dart format gate and fix the one drifted test file

Format drift was unchecked in CI. Add the gate before analyze and include integration_test in the documented format commands.

.github/workflows/ci.yml

@@ -118,6 +118,9 @@ jobs:
       - name: Get Flutter dependencies
         run: flutter pub get
 
+      - name: Check formatting
+        run: dart format --output=none --set-exit-if-changed lib test integration_test
+
       - name: Analyze
         run: flutter analyze