fix(alarmas): scope schedule-skip exceptions to skipNext only

ExcepcionAlarma._esValida matched ANY exception tipo against an
occurrence, treating it as a user skip. Only the 'skipNext' tipo
existed until now, but the next commits reuse the same model to
record scheduling-reliability failures per alarm (so the alarms list
can surface them via ultimaExcepcionPara) -- without this guard, a
recorded failure would be silently treated as if the user asked to
skip that occurrence, corrupting scheduling. Adds tipo constants to
ExcepcionAlarma for the upcoming failure kinds.

test/servicios/servicio_programacion_alarmas_test.dart

@@ -75,6 +75,42 @@ void main() {
       expect(proxima, DateTime(2026, 5, 23, 9));
     });
 
+    test(
+      'un registro de fallo de programacion NO omite esa ejecucion (solo '
+      'skipNext debe hacerlo)',
+      () {
+        final alarma = AlarmaMusical(
+          id: 'a4',
+          nombre: 'Diaria',
+          hora: 9,
+          minuto: 0,
+          tipoProgramacion: TipoProgramacionAlarma.diaria,
+          diasSemana: const [],
+        );
+        final ocurrencia = DateTime(2026, 5, 22, 9);
+
+        final proxima = servicio.calcularProxima(
+          alarma: alarma,
+          desde: DateTime(2026, 5, 22, 8),
+          excepciones: [
+            ExcepcionAlarma(
+              alarmaId: 'a4',
+              ejecucion: ocurrencia,
+              tipo: ExcepcionAlarma.tipoFalloProgramacion,
+            ),
+          ],
+        );
+
+        expect(
+          proxima,
+          ocurrencia,
+          reason:
+              'un fallo de programacion registrado no debe comportarse '
+              'como un salto de usuario',
+        );
+      },
+    );
+
     test('snooze solo permite 3, 5 o 10 minutos y cae a 5', () {
       expect(
         servicio.calcularSnooze(DateTime(2026, 5, 21, 7), 10),