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.

src/Yavsc.Server/Services/TestMailSender.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Identity.UI.Services;
+using Microsoft.Extensions.Logging;
+using Yavsc.Interface;
+
+namespace Yavsc.Services
+{
+    public class TestMailSender : ITrueEmailSender, IEmailSender
+    {
+        private readonly ILogger<TestMailSender> logger;
+
+        public TestMailSender(ILoggerFactory loggerFactory)
+        {
+            logger = loggerFactory.CreateLogger<TestMailSender>();
+        }
+
+        public Task SendEmailAsync(string email, string subject, string htmlMessage)
+        {
+            logger.LogInformation("[TestMailSender] SendEmailAsync to {Email} subject={Subject}", email, subject);
+            return Task.CompletedTask;
+        }
+
+        public Task<string> SendEmailAsync(string name, string email, string subject, string htmlMessage)
+        {
+            logger.LogInformation("[TestMailSender] SendEmailAsync to {Email} subject={Subject} name={Name}", email, subject, name);
+            return Task.FromResult($"test-message-{Guid.NewGuid()}");
+        }
+    }
+}