feat(medios,secciones): application layer + handlers TDD — ADM-001 B3+B4

- IMedioRepository, ISeccionRepository interfaces
- MediosQuery, SeccionesQuery common records
- TipoSeccion static AllowedTipos helper
- Medios: 6 use cases (Create/Update/Deactivate/Reactivate/List/GetById) with validators, handlers and DTOs
- Secciones: 6 use cases mirroring Medios; Create validates MedioId active via IMedioRepository
- 52 unit tests (xUnit + NSubstitute) all green; audit LogAsync asserted per mutating handler
- DI registrations for all 12 handlers and validators auto-scanned via AddValidatorsFromAssemblyContaining

tests/SIGCM2.Application.Tests/Secciones/Create/CreateSeccionCommandHandlerTests.cs

@@ -0,0 +1,113 @@
+using NSubstitute;
+using SIGCM2.Application.Abstractions.Persistence;
+using SIGCM2.Application.Audit;
+using SIGCM2.Application.Secciones.Create;
+using SIGCM2.Domain.Entities;
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Application.Tests.Secciones.Create;
+
+public class CreateSeccionCommandHandlerTests
+{
+    private readonly ISeccionRepository _repo = Substitute.For<ISeccionRepository>();
+    private readonly IMedioRepository _medioRepo = Substitute.For<IMedioRepository>();
+    private readonly IAuditLogger _audit = Substitute.For<IAuditLogger>();
+    private readonly CreateSeccionCommandHandler _handler;
+
+    private static Medio MakeMedio(int id = 1, bool activo = true) =>
+        new(id, "COD" + id, "Medio " + id, TipoMedio.Diario, null, activo, DateTime.UtcNow, null);
+
+    private static CreateSeccionCommand ValidCommand() => new(
+        MedioId: 1,
+        Codigo: "CLASIFICADOS_LUNES",
+        Nombre: "Clasificados Lunes",
+        Tipo: "clasificados");
+
+    public CreateSeccionCommandHandlerTests()
+    {
+        _handler = new CreateSeccionCommandHandler(_repo, _medioRepo, _audit);
+        _medioRepo.GetByIdAsync(1, Arg.Any<CancellationToken>()).Returns(MakeMedio(1));
+        _repo.ExistsByCodigoInMedioAsync(Arg.Any<int>(), Arg.Any<string>(), Arg.Any<CancellationToken>()).Returns(false);
+        _repo.AddAsync(Arg.Any<Seccion>(), Arg.Any<CancellationToken>()).Returns(10);
+    }
+
+    // ── medio not found → throws ─────────────────────────────────────────────
+
+    [Fact]
+    public async Task Handle_MedioNotFound_ThrowsMedioNotFoundException()
+    {
+        _medioRepo.GetByIdAsync(1, Arg.Any<CancellationToken>()).Returns((Medio?)null);
+
+        await Assert.ThrowsAsync<MedioNotFoundException>(
+            () => _handler.Handle(ValidCommand()));
+    }
+
+    // ── duplicate codigo in medio → throws ──────────────────────────────────
+
+    [Fact]
+    public async Task Handle_DuplicateCodigoInMedio_ThrowsSeccionCodigoDuplicadoEnMedioException()
+    {
+        _repo.ExistsByCodigoInMedioAsync(1, "CLASIFICADOS_LUNES", Arg.Any<CancellationToken>()).Returns(true);
+
+        await Assert.ThrowsAsync<SeccionCodigoDuplicadoEnMedioException>(
+            () => _handler.Handle(ValidCommand()));
+    }
+
+    [Fact]
+    public async Task Handle_DuplicateCodigo_DoesNotCallAddAsync()
+    {
+        _repo.ExistsByCodigoInMedioAsync(Arg.Any<int>(), Arg.Any<string>(), Arg.Any<CancellationToken>()).Returns(true);
+
+        try { await _handler.Handle(ValidCommand()); } catch (SeccionCodigoDuplicadoEnMedioException) { }
+
+        await _repo.DidNotReceive().AddAsync(Arg.Any<Seccion>(), Arg.Any<CancellationToken>());
+    }
+
+    // ── happy path ───────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task Handle_HappyPath_ReturnsDtoWithIdFromRepository()
+    {
+        _repo.AddAsync(Arg.Any<Seccion>(), Arg.Any<CancellationToken>()).Returns(10);
+
+        var result = await _handler.Handle(ValidCommand());
+
+        Assert.Equal(10, result.Id);
+    }
+
+    [Fact]
+    public async Task Handle_HappyPath_DtoContainsCorrectFields()
+    {
+        var result = await _handler.Handle(ValidCommand());
+
+        Assert.Equal(1, result.MedioId);
+        Assert.Equal("CLASIFICADOS_LUNES", result.Codigo);
+        Assert.Equal("Clasificados Lunes", result.Nombre);
+        Assert.Equal("clasificados", result.Tipo);
+        Assert.True(result.Activo);
+    }
+
+    [Fact]
+    public async Task Handle_HappyPath_CallsAuditWithCreateAction()
+    {
+        _repo.AddAsync(Arg.Any<Seccion>(), Arg.Any<CancellationToken>()).Returns(10);
+
+        await _handler.Handle(ValidCommand());
+
+        await _audit.Received(1).LogAsync(
+            action: "seccion.create",
+            targetType: "Seccion",
+            targetId: "10",
+            metadata: Arg.Any<object?>(),
+            ct: Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task Handle_InactiveMedio_ThrowsMedioNotFoundException()
+    {
+        _medioRepo.GetByIdAsync(1, Arg.Any<CancellationToken>()).Returns(MakeMedio(1, false));
+
+        await Assert.ThrowsAsync<MedioNotFoundException>(
+            () => _handler.Handle(ValidCommand()));
+    }
+}