refactor: migrate to kotlinx serialization, eliminate gson dependency

app/src/main/java/com/bitcointxoko/gudariwallet/api/Adapters.kt

@@ -1,9 +1,16 @@
 package com.bitcointxoko.gudariwallet.api
 
-import com.google.gson.TypeAdapter
-import com.google.gson.stream.JsonReader
-import com.google.gson.stream.JsonToken
-import com.google.gson.stream.JsonWriter
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.KSerializer
+import kotlinx.serialization.descriptors.SerialDescriptor
+import kotlinx.serialization.descriptors.buildClassSerialDescriptor
+import kotlinx.serialization.encoding.Decoder
+import kotlinx.serialization.encoding.Encoder
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonDecoder
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.jsonPrimitive
 
 /**
  * LNbits returns `comment` in PaymentExtra as either:
@@ -11,31 +18,29 @@ import com.google.gson.stream.JsonWriter
  *   - a JSON array    ["Thanks!"]
  *   - null
  *
- * Registered centrally in [GsonProvider] — but also applied via @JsonAdapter
- * on the field for extra safety.
+ * Mirrors the behaviour of the old Gson FlexibleStringAdapter.
  */
-class FlexibleStringAdapter : TypeAdapter<String?>() {
+object FlexibleStringSerializer : KSerializer<String?> {
 
-    override fun write(out: JsonWriter, value: String?) {
-        if (value == null) out.nullValue() else out.value(value)
+    override val descriptor: SerialDescriptor =
+        buildClassSerialDescriptor("FlexibleString")
+
+    @OptIn(ExperimentalSerializationApi::class)
+    override fun serialize(encoder: Encoder, value: String?) {
+        if (value == null) encoder.encodeNull()
+        else encoder.encodeString(value)
     }
 
-    override fun read(reader: JsonReader): String? {
-        return when (reader.peek()) {
-            JsonToken.NULL -> { reader.nextNull(); null }
-            JsonToken.BEGIN_ARRAY -> {
-                val parts = mutableListOf<String>()
-                reader.beginArray()
-                while (reader.hasNext()) {
-                    when (reader.peek()) {
-                        JsonToken.NULL -> reader.nextNull()
-                        else           -> parts.add(reader.nextString())
-                    }
-                }
-                reader.endArray()
-                parts.joinToString(", ").ifEmpty { null }
-            }
-            else -> reader.nextString()
+    override fun deserialize(decoder: Decoder): String? {
+        val jsonDecoder = decoder as JsonDecoder
+        return when (val element = jsonDecoder.decodeJsonElement()) {
+            is JsonNull -> null
+            is JsonPrimitive -> element.content                  // plain string → as-is
+            is JsonArray -> element                          // array → join elements
+                .mapNotNull { if (it is JsonNull) null else it.jsonPrimitive.content }
+                .joinToString(", ")
+                .ifEmpty { null }
+            else             -> null
         }
     }
-}
+}