Enable RuboCop on CI

When running `bundle exec rake` in the local environment, RuboCop is executed,
but it was not included in the CI workflow.

This PR enables RuboCop execution in the CI workflow. RuboCop performs static analysis
on the source code using Ruby 3.2, in accordance with the gemspec setting `spec.required_ruby_version = ">= 3.2.0"`:
6b49222edb/model_context_protocol.gemspec (L16)

In other words, it is not necessary to run RuboCop under every Ruby version from 3.2 to head,
so it is defined as a separate job from the test job.

.github/workflows/ci.yml

@@ -22,3 +22,14 @@ jobs:
         run: bundle install
       - run: bundle exec rake test
         continue-on-error: ${{ matrix.entry.allowed-failure }}
+
+  rubocop:
+    runs-on: ubuntu-latest
+    name: RuboCop
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 3.2 # Specify the oldest supported Ruby version.
+          bundler-cache: true
+      - run: bundle exec rake rubocop