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);
 }