feat(domain): Rubro entity + domain exceptions (CAT-001)

src/api/SIGCM2.Domain/Exceptions/RubroTieneHijosActivosException.cs

@@ -0,0 +1,17 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when attempting to soft-delete a Rubro that still has active children. → HTTP 409
+/// </summary>
+public sealed class RubroTieneHijosActivosException : DomainException
+{
+    public int Id { get; }
+    public int Count { get; }
+
+    public RubroTieneHijosActivosException(int id, int count)
+        : base($"El rubro con id '{id}' tiene {count} subrubros activos.")
+    {
+        Id = id;
+        Count = count;
+    }
+}