feat: tested and improved GameState

lib/game/bloc/game_state.dart

@@ -1,14 +1,25 @@
 part of 'game_bloc.dart';
 
+/// {@template game_state}
+/// Represents the state of the pinball game.
+/// {@endtemplate}
 class GameState extends Equatable {
+  /// {@macro game_state}
   const GameState({
     required this.score,
     required this.balls,
-  });
+  })  : assert(score >= 0, "Score can't be negative"),
+        assert(balls >= 0, "Number of balls can't be negative");
 
+  /// The current score of the game.
   final int score;
+
+  /// The number of balls left in the game.
+  ///
+  /// When the number of balls is 0, the game is over.
   final int balls;
 
+  /// Determines when the game is over.
   bool get isGameOver => balls == 0;
 
   GameState copyWith({

test/game/bloc/game_state_test.dart

@@ -0,0 +1,63 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pinball/game/game.dart';
+
+void main() {
+  group('GameState', () {
+    test('supports value equality', () {
+      expect(
+        const GameState(score: 0, balls: 0),
+        equals(const GameState(score: 0, balls: 0)),
+      );
+    });
+
+    group('constructor', () {
+      test('can be instantiated', () {
+        expect(const GameState(score: 0, balls: 0), isNotNull);
+      });
+    });
+
+    test(
+      'throws AssertionError '
+      'when balls are negative',
+      () {
+        expect(
+          () => GameState(balls: -1, score: 0),
+          throwsAssertionError,
+        );
+      },
+    );
+
+    test(
+      'throws AssertionError '
+      'when score is negative',
+      () {
+        expect(
+          () => GameState(balls: 0, score: -1),
+          throwsAssertionError,
+        );
+      },
+    );
+  });
+
+  group('isGameOver', () {
+    test(
+        'is true '
+        'when no balls are left', () {
+      const gameState = GameState(
+        balls: 0,
+        score: 0,
+      );
+      expect(gameState.isGameOver, isTrue);
+    });
+
+    test(
+        'is false '
+        'when one 1 ball left', () {
+      const gameState = GameState(
+        balls: 1,
+        score: 0,
+      );
+      expect(gameState.isGameOver, isFalse);
+    });
+  });
+}