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/GetById/GetSeccionByIdQueryHandlerTests.cs

@@ -0,0 +1,46 @@
+using NSubstitute;
+using SIGCM2.Application.Abstractions.Persistence;
+using SIGCM2.Application.Secciones.GetById;
+using SIGCM2.Domain.Entities;
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Application.Tests.Secciones.GetById;
+
+public class GetSeccionByIdQueryHandlerTests
+{
+    private readonly ISeccionRepository _repo = Substitute.For<ISeccionRepository>();
+    private readonly GetSeccionByIdQueryHandler _handler;
+
+    public GetSeccionByIdQueryHandlerTests()
+    {
+        _handler = new GetSeccionByIdQueryHandler(_repo);
+    }
+
+    private static Seccion MakeSeccion(int id) =>
+        new(id, 2, "COD" + id, "Nombre " + id, "suplementos", true, DateTime.UtcNow, null);
+
+    [Fact]
+    public async Task Handle_NotFound_ThrowsSeccionNotFoundException()
+    {
+        _repo.GetByIdAsync(999, Arg.Any<CancellationToken>()).Returns((Seccion?)null);
+
+        await Assert.ThrowsAsync<SeccionNotFoundException>(
+            () => _handler.Handle(new GetSeccionByIdQuery(999)));
+    }
+
+    [Fact]
+    public async Task Handle_HappyPath_ReturnsDtoWithCorrectFields()
+    {
+        var seccion = MakeSeccion(5);
+        _repo.GetByIdAsync(5, Arg.Any<CancellationToken>()).Returns(seccion);
+
+        var result = await _handler.Handle(new GetSeccionByIdQuery(5));
+
+        Assert.Equal(5, result.Id);
+        Assert.Equal(2, result.MedioId);
+        Assert.Equal("COD5", result.Codigo);
+        Assert.Equal("Nombre 5", result.Nombre);
+        Assert.Equal("suplementos", result.Tipo);
+        Assert.True(result.Activo);
+    }
+}