Add a spec to cover CheckRegistry changes
user
```ruby

RSpec.describe Rack::ECG::CheckRegistry do
  class MyCheckClass; end
  subject(:check_registry) { described_class }

  before do
    check_registry.register(:my_check, MyCheckClass)
  end

  describe ".lookup" do
    context "with a registered class" do
      it "returns the registered class" do
        expect(check_registry.lookup(:my_check)).to eq(MyCheckClass)
      end
    end

    context "when the class is not registered" do
      it "raises an error" do
        expect { check_registry.lookup(:my_other_check) }.to raise_error(Rack::ECG::CheckRegistry::CheckNotRegistered)
      end
    end
  end
end
```
assistant