fix(devices): cache platform device names, dedupe placeholder ids, purge collided EQ entries

Dart half of bt-device-identity. EstadoEcualizador now caches each
device's platform-reported name in memory so the settings screen
shows the device's own Bluetooth name instead of its raw id when no
custom rename exists, and skips auto-creating preset entries for the
composite-placeholder sentinel. Enabling multi-device EQ triggers the
Bluetooth permission request through the new channel contract. A
flag-guarded one-time migration purges only entries keyed by the
exact literal placeholder id from the three per-device preference
maps, since those collided entries cannot be attributed to a device.

Work unit 2/2 of bt-device-identity (Dart state + migration).

test/servicios/servicio_dispositivo_audio_real_test.dart

@@ -37,6 +37,16 @@ void main() {
           });
     }
 
+    void stubRequestBluetoothConnect(bool? response) {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+          .setMockMethodCallHandler(const MethodChannel(methodChannelName), (
+            call,
+          ) async {
+            if (call.method == 'requestBluetoothConnect') return response;
+            return null;
+          });
+    }
+
     test('obtenerDispositivoActual maps builtin_speaker (type=2)', () async {
       stubGetActiveDevice({
         'id': 'builtin_speaker',
@@ -105,5 +115,36 @@ void main() {
     test('onDispositivoCambiado is a broadcast stream', () {
       expect(servicio.onDispositivoCambiado.isBroadcast, isTrue);
     });
+
+    // ── bt-device-identity Phase 2: permission contract ──────────────────────
+
+    test(
+      'solicitarPermisoBluetooth invokes requestBluetoothConnect and '
+      'returns true when granted',
+      () async {
+        stubRequestBluetoothConnect(true);
+        final granted = await servicio.solicitarPermisoBluetooth();
+        expect(granted, isTrue);
+      },
+    );
+
+    test(
+      'solicitarPermisoBluetooth returns false when the platform denies',
+      () async {
+        stubRequestBluetoothConnect(false);
+        final granted = await servicio.solicitarPermisoBluetooth();
+        expect(granted, isFalse);
+      },
+    );
+
+    test(
+      'solicitarPermisoBluetooth returns false when the platform channel '
+      'returns null',
+      () async {
+        stubRequestBluetoothConnect(null);
+        final granted = await servicio.solicitarPermisoBluetooth();
+        expect(granted, isFalse);
+      },
+    );
   });
 }