fix: test infrastructure with in-memory DB, SMTP mocking, and thread-safe billing configuration

- Add in-memory database support for test isolation in WebServerFixture
- Implement TestMailSender fake SMTP provider for email test support
- Add thread synchronization to billing service registration to prevent race conditions
- Make RegisterBilling<T> idempotent to safely handle reconfiguration
- Configure test environment via in-memory settings (UseTestEmailSender, UseInMemoryDatabase)
- Add regression tests for billing module idempotency and duplicate registration detection
- Fix tests: EMaillingTests.SendEMailSynchrone, BillingServiceTests (2 tests), HaveConfigurationRoot (3 tests)

All core test infrastructure tests now passing.

test/yavscTests/NonRegression/BillingServiceTests.cs

@@ -0,0 +1,48 @@
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+using Yavsc;
+using Yavsc.Abstract.Workflow;
+using Yavsc.Helpers;
+using Yavsc.Models;
+using Yavsc.Models.Billing;
+using Yavsc.Models.Haircut;
+using Yavsc.Services;
+
+namespace yavscTests
+{
+    [Trait("regression", "II")]
+    public class BillingServiceTests
+    {
+        [Fact]
+        public void ConfigureBillingService_CanBeCalledTwiceWithoutThrowing()
+        {
+            // First initialization should populate the billing registry.
+            WorkflowHelpers.ConfigureBillingService();
+
+            int firstBillingCount = BillingService.Billing.Count;
+            int firstSettingsCount = BillingService.UserSettings.Count;
+            int firstProfileTypesCount = Config.ProfileTypes.Count;
+
+            // Second call should be idempotent and not throw.
+            WorkflowHelpers.ConfigureBillingService();
+
+            Assert.Equal(firstBillingCount, BillingService.Billing.Count);
+            Assert.Equal(firstSettingsCount, BillingService.UserSettings.Count);
+            Assert.Equal(firstProfileTypesCount, Config.ProfileTypes.Count);
+        }
+
+        [Fact]
+        public void RegisterBilling_DuplicateRegistrationThrowsInvalidOperationException()
+        {
+            WorkflowHelpers.ConfigureBillingService();
+
+            var firstRegistrar = new Func<ApplicationDbContext, long, IDecidableQuery>((db, id) =>
+                db.HairCutQueries.Include(q => q.Prestation).Include(q => q.Regularisation).Single(q => q.Id == id));
+
+            const string testCode = "TestBrush";
+            
+            Assert.Throws<InvalidOperationException>(() =>
+                WorkflowHelpers.RegisterBilling<HairCutQuery>(testCode, firstRegistrar));
+        }
+    }
+}