refactor: migrate Gson serialization to kotlinx.serialization (#475)

* refactor: migrate Gson serialization to kotlinx.serialization

* fix(dto): resolve serializing Any? and Map<String, Any?> with JsonElement

* test(refactor): fix unit test issues under kotlinx.serialization and update nullable DTO defaults

* fix(chat): support flexible numeric and string decoding for message timestamps

* fix(chat): sequentialize cache loading to avoid overwriting network state

app/src/main/java/com/m57/hermescontrol/data/remote/ApiClient.kt

@@ -4,9 +4,10 @@ import com.m57.hermescontrol.BuildConfig
 import com.m57.hermescontrol.data.local.AuthManager
 import okhttp3.CertificatePinner
 import okhttp3.Interceptor
+import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.logging.HttpLoggingInterceptor
 import retrofit2.Retrofit
-import retrofit2.converter.gson.GsonConverterFactory
+import retrofit2.converter.kotlinx.serialization.asConverterFactory
 
 /**
  * Provides a Retrofit-backed [HermesApiService].
@@ -71,7 +72,7 @@ object ApiClient {
                 .Builder()
                 .baseUrl("http://$host:$port/")
                 .client(tempOkHttp)
-                .addConverterFactory(GsonConverterFactory.create(OkHttpProvider.gson))
+                .addConverterFactory(OkHttpProvider.json.asConverterFactory("application/json".toMediaType()))
                 .build()
 
         return tempRetrofit.create(HermesApiService::class.java)
@@ -135,7 +136,7 @@ object ApiClient {
                 .Builder()
                 .baseUrl(AuthManager.baseUrl())
                 .client(okHttp)
-                .addConverterFactory(GsonConverterFactory.create(OkHttpProvider.gson))
+                .addConverterFactory(OkHttpProvider.json.asConverterFactory("application/json".toMediaType()))
                 .build()
                 .also { retrofit = it }
 

app/src/main/java/com/m57/hermescontrol/data/remote/HermesApiService.kt

@@ -1,6 +1,5 @@
 package com.m57.hermescontrol.data.remote
 
-import com.google.gson.JsonElement
 import com.m57.hermescontrol.data.model.AchievementsResponse
 import com.m57.hermescontrol.data.model.ActionResponse
 import com.m57.hermescontrol.data.model.ActionStatusResponse
@@ -91,6 +90,7 @@ import com.m57.hermescontrol.data.model.WebhookSubscription
 import com.m57.hermescontrol.data.model.WebhookToggleSubscriptionRequest
 import com.m57.hermescontrol.data.model.WebhooksResponse
 import com.m57.hermescontrol.data.model.WebhooksToggleRequest
+import kotlinx.serialization.json.JsonElement
 import okhttp3.ResponseBody
 import retrofit2.Response
 import retrofit2.http.Body

app/src/main/java/com/m57/hermescontrol/data/remote/NetworkResult.kt

@@ -1,7 +1,8 @@
 package com.m57.hermescontrol.data.remote
 
-import com.google.gson.JsonParser
 import kotlinx.coroutines.delay
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
 import retrofit2.Response
 import java.io.IOException
 
@@ -49,11 +50,8 @@ fun mapHttpError(
     val detail =
         if (errorBody != null) {
             try {
-                val parsed =
-                    com.google.gson.JsonParser
-                        .parseString(errorBody)
-                        .asJsonObject
-                parsed.get("detail")?.asString
+                val parsed = OkHttpProvider.json.parseToJsonElement(errorBody).jsonObject
+                parsed["detail"]?.jsonPrimitive?.content
             } catch (_: Exception) {
                 null
             }

app/src/main/java/com/m57/hermescontrol/data/remote/OkHttpProvider.kt

@@ -1,7 +1,8 @@
 package com.m57.hermescontrol.data.remote
 
-import com.google.gson.Gson
-import com.google.gson.GsonBuilder
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonNamingStrategy
 import okhttp3.ConnectionPool
 import okhttp3.OkHttpClient
 import java.util.concurrent.TimeUnit
@@ -42,6 +43,10 @@ object OkHttpProvider {
             .followRedirects(false)
             .build()
 
-    // Shared Gson — thread-safe, zero-config
-    val gson: Gson = GsonBuilder().create()
+    @OptIn(ExperimentalSerializationApi::class)
+    val json: Json =
+        Json {
+            ignoreUnknownKeys = true
+            namingStrategy = JsonNamingStrategy.SnakeCase
+        }
 }