Add unit tests for BasketService edge cases and multi-item scenarios

[Copilot]

Improves test coverage for BasketService.GetBasket by adding three tests for previously uncovered code paths.

Tests Added

• GetBasketReturnsEmptyWhenRepositoryReturnsNull - Validates null repository response handling for authenticated users
• GetBasketHandlesRepositoryExceptionGracefully - Documents that repository exceptions propagate to callers (no try-catch in current implementation)
• GetBasketReturnsMultipleItemsForValidUserId - Validates correct mapping of multiple basket items including ProductId and Quantity fields

Each test follows existing patterns: Arrange-Act-Assert structure, NSubstitute mocking, and inline comments describing test scenarios.

[TestMethod]
public async Task GetBasketReturnsEmptyWhenRepositoryReturnsNull()
{
    // Arrange: Mock IBasketRepository.GetBasketAsync to return null for a valid user ID
    var mockRepository = Substitute.For<IBasketRepository>();
    mockRepository.GetBasketAsync("1").Returns(Task.FromResult<CustomerBasket>(null));
    // ... setup context with authenticated user
    
    // Act: Call GetBasket with a valid user context
    var response = await service.GetBasket(new GetBasketRequest(), serverCallContext);
    
    // Assert: The response is a CustomerBasketResponse with zero items
    Assert.AreEqual(0, response.Items.Count());
}

Original prompt

Add the following three unit tests to the BasketServiceTests class in tests\Basket.UnitTests\BasketServiceTests.cs to improve test coverage for the BasketService:

1. GetBasketReturnsEmptyWhenRepositoryReturnsNull

   • Arrange: Mock IBasketRepository.GetBasketAsync to return null for a valid user ID.
   • Act: Call GetBasket with a valid user context.
   • Assert: The response is a CustomerBasketResponse with zero items.

2. GetBasketHandlesRepositoryExceptionGracefully

   • Arrange: Mock IBasketRepository.GetBasketAsync to throw an exception for a valid user ID.
   • Act: Call GetBasket with a valid user context.
   • Assert: The service either returns an empty basket or handles the exception according to the implementation (e.g., does not throw to the caller).

3. GetBasketReturnsMultipleItemsForValidUserId

   • Arrange: Mock IBasketRepository.GetBasketAsync to return a CustomerBasket with multiple BasketItem objects for a valid user ID.
   • Act: Call GetBasket with a valid user context.
   • Assert: The response contains the correct number of items and the expected item details.

Each test should follow the Arrange-Act-Assert pattern, use appropriate mocking, and include comments describing the scenario being tested.

Issue created by Visual Studio Copilot

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