feat(application): DeactivateRubroCommandHandler guard contra Products activos

Extiende IProductQueryRepository con CountActiveByRubroAsync, inyecta
el repositorio en el handler e intercala el chequeo después del guard
de hijos activos. Tests de unidad cubren: throw, success con 0 productos,
y estabilidad del orden de guardas (hijos primero).

src/api/SIGCM2.Application/Abstractions/Persistence/IProductQueryRepository.cs

@@ -12,4 +12,10 @@ public interface IProductQueryRepository
     /// Used by DeactivateProductTypeCommandHandler to guard against orphaning active products.
     /// </summary>
     Task<bool> ExistsActiveByProductTypeAsync(int productTypeId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns the count of active Products where RubroId = rubroId.
+    /// Used by DeactivateRubroCommandHandler to guard against orphaning active products. (issue #41)
+    /// </summary>
+    Task<int> CountActiveByRubroAsync(int rubroId, CancellationToken ct = default);
 }

src/api/SIGCM2.Application/Rubros/Deactivate/DeactivateRubroCommandHandler.cs

@@ -10,15 +10,18 @@ public sealed class DeactivateRubroCommandHandler : ICommandHandler<DeactivateRu
 {
     private readonly IRubroRepository _repo;
     private readonly IAuditLogger _audit;
+    private readonly IProductQueryRepository _productQuery;
     private readonly TimeProvider _timeProvider;
 
     public DeactivateRubroCommandHandler(
         IRubroRepository repo,
         IAuditLogger audit,
+        IProductQueryRepository productQuery,
         TimeProvider timeProvider)
     {
         _repo = repo;
         _audit = audit;
+        _productQuery = productQuery;
         _timeProvider = timeProvider;
     }
 
@@ -31,6 +34,10 @@ public sealed class DeactivateRubroCommandHandler : ICommandHandler<DeactivateRu
         if (activeChildren > 0)
             throw new RubroTieneHijosActivosException(command.Id, activeChildren);
 
+        var productosActivos = await _productQuery.CountActiveByRubroAsync(command.Id);
+        if (productosActivos > 0)
+            throw new RubroConProductosActivosException(command.Id, productosActivos);
+
         var deactivated = target.WithActivo(false, _timeProvider);
 
         using var tx = new TransactionScope(