fix(auto): restore the equalizer toggle and list the user's own presets

Two Android Auto regressions reported from the car.

1. The on/off equalizer action disappeared from the playback screen.

That was self-inflicted: commit cacd3ec removed it on the theory that a
custom action in `controls` aborts `AudioService.setState` and kills the
media notification. Reading the plugin source refutes it. setState
(AudioService.java:513-520) SPLITS the list -- a control carrying a
customAction goes to `customActions` (PlaybackStateCompat, i.e. the car),
everything else becomes a NotificationCompat.Action in `nativeActions`
(the phone notification). The two never mix. And the throw the theory
depended on cannot happen here: ic_auto_eq_on/ic_auto_eq_off both exist
under res/drawable, and the labels are non-empty in all 13 locales.

The notification outage was already fixed by abc6b47 (transient idle on
a source change, which setState turns into a full stop() at :557).

The action is back, with both state-aware icons. The real invariant --
a custom action's icon must resolve and its label must be non-empty --
is now a test that reads res/drawable and fails on a missing file,
instead of a comment claiming custom actions are forbidden outright.

2. The Ecualizador folder never listed the user's saved presets.

itemsEcualizadorAuto iterated PresetEcualizador.presets, so only the six
factory presets appeared -- the user's own were unreachable from the
car, the surface where a preset picker matters most. They now arrive
through a registered read function (same seam as stations and local
music, re-read per browse so a preset saved on the phone shows up
without an app restart).

presetsEcualizadorAuto is the single source of truth for the ordered
universe, used to BUILD the items and to RESOLVE a tap, so the folder
cannot show an item that resolution then refuses -- which is what the
factory-only default in seleccionarPresetEqPorMediaId would have caused.
A custom preset whose name collides with a factory one is dropped: the
media id is the raw name, so it could only ever resolve to the factory
entry, and an item that applies a preset other than the one it names is
worse than an absent one.

Tests: 1108 -> 1120.

test/servicios/servicio_audio_eq_folder_test.dart

@@ -120,4 +120,135 @@ void main() {
       expect(desactivar.title, contains(l10n.autoEqDisableOption));
     });
   });
+
+  /// On-device feedback: the folder only ever listed the six FACTORY
+  /// presets, so a user's own saved presets were unreachable from the car —
+  /// the surface where a preset picker matters most, since you cannot pull
+  /// out the phone and drag five band sliders while driving.
+  group('presets personalizados del usuario en la carpeta', () {
+    final mio = PresetEcualizador(
+      nombre: 'Mi coche',
+      bandas: [4.0, 2.0, 0.0, 1.0, 3.0],
+    );
+    final otro = PresetEcualizador(
+      nombre: 'Podcast noche',
+      bandas: [-3.0, 0.0, 3.0, 2.0, -1.0],
+    );
+
+    test('se listan tras los 6 de fábrica, en orden de guardado', () {
+      final items = itemsEcualizadorAuto(
+        activo: true,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+        presetsPersonalizados: [mio, otro],
+      );
+
+      expect(items, hasLength(9));
+      expect(items[7].title, contains('Mi coche'));
+      expect(items[8].title, contains('Podcast noche'));
+    });
+
+    test('su id sigue el mismo formato eq_preset:<nombre> y resuelve al '
+        'preset correcto con la lista combinada', () {
+      final constructor = ConstructorArbolAuto();
+      final universo = presetsEcualizadorAuto(personalizados: [mio]);
+      final items = itemsEcualizadorAuto(
+        activo: true,
+        presetActual: PresetEcualizador.flat,
+        l10n: l10n,
+        presetsPersonalizados: [mio],
+      );
+
+      final item = items.last;
+      expect(item.id, constructor.idPresetEq('Mi coche'));
+      expect(item.playable, isTrue);
+      expect(constructor.resolverPresetEq(item.id, presets: universo), mio);
+    });
+
+    test('sin la lista combinada NO resuelve: es exactamente el fallo que se '
+        'reportó, un item visible que al pulsarlo no hacía nada', () {
+      final constructor = ConstructorArbolAuto();
+
+      // Default `presets` == the factory six only. This is what the tap
+      // dispatch used to do, and why a listed custom preset was a no-op.
+      expect(
+        constructor.resolverPresetEq(constructor.idPresetEq('Mi coche')),
+        isNull,
+      );
+    });
+
+    test('pulsar un preset personalizado lo aplica de verdad', () async {
+      final aplicados = <PresetEcualizador>[];
+      var encendido = false;
+
+      await seleccionarPresetEqPorMediaId(
+        ConstructorArbolAuto().idPresetEq('Mi coche'),
+        activo: false,
+        aplicarPreset: (p) async => aplicados.add(p),
+        activarEcualizador: (v) async => encendido = v,
+        presets: presetsEcualizadorAuto(personalizados: [mio]),
+      );
+
+      expect(aplicados, [mio]);
+      // Tapping any preset while the EQ is off must also switch it on,
+      // exactly as it already does for a factory preset.
+      expect(encendido, isTrue);
+    });
+
+    test('el preset personalizado activo se marca con ✓, y solo él', () {
+      final items = itemsEcualizadorAuto(
+        activo: true,
+        presetActual: mio,
+        l10n: l10n,
+        presetsPersonalizados: [mio, otro],
+      );
+
+      final marcados = items.where((i) => i.title.contains('✓')).toList();
+      expect(marcados, hasLength(1));
+      expect(marcados.single.id, ConstructorArbolAuto().idPresetEq('Mi coche'));
+    });
+
+    test('un personalizado que repite el nombre de uno de fábrica se DESCARTA '
+        '— su id solo podría resolver al de fábrica, así que mostrarlo '
+        'aplicaría un preset distinto del que anuncia', () {
+      final impostor = PresetEcualizador(
+        nombre: 'Rock',
+        bandas: [9.0, 9.0, 9.0, 9.0, 9.0],
+      );
+
+      final universo = presetsEcualizadorAuto(personalizados: [impostor, mio]);
+
+      expect(universo.where((p) => p.nombre == 'Rock'), hasLength(1));
+      expect(
+        universo.firstWhere((p) => p.nombre == 'Rock'),
+        PresetEcualizador.rock,
+      );
+      expect(universo, contains(mio));
+      expect(
+        itemsEcualizadorAuto(
+          activo: true,
+          presetActual: PresetEcualizador.flat,
+          l10n: l10n,
+          presetsPersonalizados: [impostor, mio],
+        ),
+        hasLength(8),
+      );
+    });
+
+    test('sin presets guardados la carpeta queda exactamente como antes', () {
+      expect(
+        itemsEcualizadorAuto(
+          activo: true,
+          presetActual: PresetEcualizador.flat,
+          l10n: l10n,
+        ).map((i) => i.id),
+        itemsEcualizadorAuto(
+          activo: true,
+          presetActual: PresetEcualizador.flat,
+          l10n: l10n,
+          presetsPersonalizados: const [],
+        ).map((i) => i.id),
+      );
+    });
+  });
 }