feat(api): UDT-003 registro de usuarios — backend completo (Phases 1-6)

- Domain: Usuario.ForCreation factory, UsernameAlreadyExistsException, IUsuarioRepository extendido
- Application: CreateUsuarioCommand/Validator/Handler, UsuarioCreatedDto, AuthOptions password policy
- Infrastructure: UsuarioRepository.ExistsByUsernameAsync + AddAsync (INSERT OUTPUT INSERTED.Id), RoleClaimType="rol" en TokenValidationParameters
- Api: UsuariosController POST api/v1/users [Authorize(Roles="admin")], ExceptionFilter mapea UsernameAlreadyExistsException + SqlException 2627 → 409
- Tests (unit): 43 tests — 33 validator + 10 handler (107 total, green)
- Tests (integration): 7 tests CreateUsuarioEndpoint — 401/403/400/201/409/race/e2e (green)
- Fix: TestWebAppFactory.ConfigureTestServices reemplaza SqlConnectionFactory singleton con CS de test correcto

tests/SIGCM2.TestSupport/TestWebAppFactory.cs

@@ -1,9 +1,11 @@
 using System.Security.Cryptography;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using SIGCM2.Application.Abstractions.Security;
+using SIGCM2.Infrastructure.Persistence;
 using SIGCM2.Infrastructure.Security;
 using Xunit;
 
@@ -49,6 +51,22 @@ public sealed class TestWebAppFactory : WebApplicationFactory<Program>, IAsyncLi
         });
 
         builder.UseEnvironment("Testing");
+
+        // Step 2: Replace SqlConnectionFactory singleton with the correct test connection.
+        // ConfigureAppConfiguration alone is insufficient because WebApplication.CreateBuilder
+        // evaluates configuration for singleton construction before overrides apply.
+        // ConfigureTestServices runs AFTER all services are registered, so it wins.
+        builder.ConfigureTestServices(services =>
+        {
+            // Remove the existing SqlConnectionFactory singleton registered by AddInfrastructure
+            var descriptor = services.SingleOrDefault(
+                d => d.ServiceType == typeof(SqlConnectionFactory));
+            if (descriptor is not null)
+                services.Remove(descriptor);
+
+            // Re-register with the test connection string
+            services.AddSingleton(new SqlConnectionFactory(TestConnectionString));
+        });
     }
 
     public async Task InitializeAsync()