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));
+        }
+    }
+}

test/yavscTests/WebServerFixture.cs

@@ -28,7 +28,7 @@ namespace isnd.tests
 
         private SiteSettings siteSettings;
 
-        public IConfigurationRoot Configuration { get; private set; }
+        public IConfiguration Configuration { get; private set; }
 
         private WebApplication app;
         public string TestClientId { get; private set; }
@@ -73,12 +73,22 @@ namespace isnd.tests
         {
 
             var builder = WebApplication.CreateBuilder();
+            builder.Environment.EnvironmentName = "Development";
             ConfigureLogger();
-            Configuration = builder.Configuration
-                .AddJsonFile("appsettings.json")
-                .AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true)
+            builder.Configuration
+                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: false)
+                .AddJsonFile($"appsettings.{builder.Environment.EnvironmentName}.json", optional: true, reloadOnChange: false)
                 .AddEnvironmentVariables()
-                .Build();
+                .AddInMemoryCollection(new Dictionary<string, string?>
+                {
+                    ["UseInMemoryDatabase"] = "true",
+                    ["UseTestEmailSender"] = "true",
+                    ["Smtp:Host"] = "localhost",
+                    ["Smtp:Port"] = "25",
+                    ["Smtp:SenderName"] = "Yavsc Test",
+                    ["Smtp:SenderEmail"] = "test@example.com"
+                });
+            Configuration = builder.Configuration;
 
             this.app = builder.ConfigureWebAppServices();
             Services = app.Services;

test/yavscTests/appsettings.json

@@ -41,7 +41,7 @@
         }
     },
     "ConnectionStrings": {
-        "YavscConnection": "Server=lame-NpgsqlHostName;Port=5432;Database=lame-DataBase;Username=lame-Username;Password=lame-dbPassword;",
+        "YavscConnection": "Server=lame-NpgsqlHostName;Port=5432;Database=lame-DataBase;Username=lame-Username;Password=lame-dbPassword;"
       },
     "DataProtection": {
         "Keys": {

test/yavscTests/yavscTests.csproj

@@ -24,6 +24,11 @@
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.visualstudio" />
   </ItemGroup>
+  <ItemGroup>
+    <Content Include="appsettings.json">
+      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Yavsc.Org\Yavsc.Org.csproj" />
     <ProjectReference Include="..\..\src\Yavsc.Abstract\Yavsc.Abstract.csproj" />