refactor: migrate to kotlinx serialization, eliminate gson dependency

app/src/main/java/com/bitcointxoko/gudariwallet/service/WalletNotificationService.kt

@@ -7,7 +7,7 @@ import android.content.pm.ServiceInfo
 import android.os.Build
 import android.os.IBinder
 import timber.log.Timber
-import com.bitcointxoko.gudariwallet.api.GsonProvider
+import com.bitcointxoko.gudariwallet.api.JsonProvider
 import com.bitcointxoko.gudariwallet.api.LNbitsClient
 import com.bitcointxoko.gudariwallet.api.PaymentRecord
 import com.bitcointxoko.gudariwallet.api.WsPaymentMessage
@@ -98,7 +98,7 @@ class WalletNotificationService : Service() {
     private var reconnectAttempts = 0
     private var currentBackoffMs = WalletConstants.WS_INITIAL_BACKOFF_MS
 
-    private val gson = GsonProvider.gson
+    private val json = JsonProvider.json
     // ── Lifecycle ─────────────────────────────────────────────────────────────
 
     override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
@@ -193,7 +193,7 @@ class WalletNotificationService : Service() {
 
     private fun handleMessage(text: String) {
         val msg = runCatching {
-            gson.fromJson(text, WsPaymentMessage::class.java)
+            json.decodeFromString(WsPaymentMessage.serializer(), text)
         }.getOrNull() ?: return
 
         val payment = msg.payment ?: return