Add test coverage for contrast calculation

src/api/spec/helpers/webui/color_helper_spec.rb

@@ -0,0 +1,15 @@
+RSpec.describe Webui::ColorHelper do
+  describe '#contrast_text' do
+    context 'when passing a light color' do
+      it 'returns a black text' do
+        expect(contrast_text('#ffffff')).to eql('black')
+      end
+    end
+
+    context 'when passing a dark color' do
+      it 'returns a white text' do
+        expect(contrast_text('#000000')).to eql('white')
+      end
+    end
+  end
+end