RED: add tests for conjured items

test/gilded_rose_test.dart

@@ -143,6 +143,38 @@ void main() {
       });
     });
 
+    group('Conjured items', () {
+      test('degrade in quality twice as fast as normal items', () {
+        final items = [Item('Conjured Mana Cake', 10, 20)];
+        final gildedRose = GildedRose(items);
+
+        gildedRose.updateQuality();
+
+        expect(items[0].quality, equals(18)); // -2 instead of -1
+        expect(items[0].sellIn, equals(9));
+      });
+
+      test('degrade twice as fast after sell-by date', () {
+        final items = [Item('Conjured Mana Cake', 0, 20)];
+        final gildedRose = GildedRose(items);
+
+        gildedRose.updateQuality();
+
+        expect(items[0].quality, equals(16)); // -4 instead of -2
+        expect(items[0].sellIn, equals(-1));
+      });
+
+      test('never have negative quality', () {
+        final items = [Item('Conjured Mana Cake', 5, 1)];
+        final gildedRose = GildedRose(items);
+
+        gildedRose.updateQuality();
+
+        expect(items[0].quality, equals(0));
+        expect(items[0].sellIn, equals(4));
+      });
+    });
+
     group('Golden Master - 30-day simulation', () {
       test('captures complete behavior of all item types over time', () {
         final items = _createAllItemTypes();