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.Org/Yavsc.Org.csproj

@@ -31,6 +31,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
+    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" />
     <PackageReference Include="Google.Apis.Compute.v1" />