Add LittleShop.Client SDK library with complete API wrapper

Features:
- Complete .NET client SDK for LittleShop API
- JWT authentication with automatic token management
- Catalog service for products and categories
- Order service with payment creation
- Retry policies using Polly for resilience
- Error handling middleware
- Dependency injection support
- Comprehensive documentation and examples

SDK Components:
- Authentication service with token refresh
- Strongly-typed models for all API responses
- HTTP handlers for retry and error handling
- Extension methods for easy DI registration
- Example console application demonstrating usage

Test Updates:
- Fixed test compilation errors
- Updated test data builders for new models
- Corrected service constructor dependencies
- Fixed enum value changes (PaymentStatus, OrderStatus)

Documentation:
- Complete project README with features and usage
- Client SDK README with detailed examples
- API endpoint documentation
- Security considerations
- Deployment guidelines

Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop.Tests/Integration/OrdersControllerTests.cs

@@ -289,7 +289,7 @@ public class OrdersControllerTests : IClassFixture<TestWebApplicationFactory>
         var webhookDto = new PaymentWebhookDto
         {
             InvoiceId = "INV123456",
-            Status = PaymentStatus.Confirmed,
+            Status = PaymentStatus.Paid,
             Amount = 100.00m,
             TransactionHash = "tx123456789"
         };
@@ -407,7 +407,7 @@ public class OrdersControllerTests : IClassFixture<TestWebApplicationFactory>
         { 
             Id = Guid.NewGuid(),
             IdentityReference = identityReference,
-            Status = OrderStatus.Pending,
+            Status = OrderStatus.PendingPayment,
             ShippingName = "Test Customer",
             ShippingAddress = "Test Address",
             ShippingCity = "Test City",