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/ui/chat/ChatBubble.kt

@@ -82,6 +82,7 @@ import androidx.compose.ui.unit.sp
 import coil.compose.AsyncImage
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.data.model.Attachment
+import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.theme.AssistantBubble
 import com.m57.hermescontrol.theme.AssistantBubbleLight
 import com.m57.hermescontrol.theme.StatusGreen
@@ -92,6 +93,14 @@ import com.m57.hermescontrol.theme.ToolChipColorLight
 import com.m57.hermescontrol.theme.UserBubble
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.launch
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonNull
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.boolean
+import kotlinx.serialization.json.double
+import kotlinx.serialization.json.int
 import java.time.Instant
 import java.time.ZoneId
 import java.time.format.DateTimeFormatter
@@ -537,9 +546,7 @@ fun parseToolOutput(
     val trimmed = content.trim()
     if (!trimmed.startsWith("{") && !trimmed.startsWith("[")) return null
     return try {
-        val element =
-            com.google.gson.JsonParser
-                .parseString(trimmed)
+        val element = OkHttpProvider.json.parseToJsonElement(trimmed)
         if (!element.isJsonObject) return null
         val obj = element.asJsonObject
 
@@ -2301,3 +2308,23 @@ private fun InlineAttachment(
         }
     }
 }
+
+// Extensions for backward compatibility with Gson API under kotlinx.serialization
+private val JsonElement.isJsonObject: Boolean get() = this is JsonObject
+private val JsonElement.asJsonObject: JsonObject get() = this as JsonObject
+private val JsonElement.isJsonArray: Boolean get() = this is JsonArray
+private val JsonElement.asJsonArray: JsonArray get() = this as JsonArray
+private val JsonElement.isJsonNull: Boolean get() = this is JsonNull
+private val JsonElement.isJsonPrimitive: Boolean get() = this is JsonPrimitive
+private val JsonElement.asString: String get() = (this as JsonPrimitive).content
+private val JsonElement.asDouble: Double get() = (this as JsonPrimitive).double
+private val JsonElement.asInt: Int get() = (this as JsonPrimitive).int
+private val JsonElement.asBoolean: Boolean get() = (this as JsonPrimitive).boolean
+
+private fun JsonElement.has(key: String): Boolean = (this as? JsonObject)?.containsKey(key) == true
+
+private fun JsonObject.entrySet(): Set<Map.Entry<String, JsonElement>> = entries
+
+private fun JsonObject.keySet(): Set<String> = keys
+
+private fun JsonArray.size(): Int = size

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -18,6 +18,7 @@ import com.m57.hermescontrol.data.ws.ConnectionStatus
 import com.m57.hermescontrol.data.ws.HermesWsClient
 import com.m57.hermescontrol.data.ws.WsEvent
 import com.m57.hermescontrol.data.ws.WsMethods
+import com.m57.hermescontrol.data.ws.toJsonElement
 import kotlinx.coroutines.CompletableDeferred
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
@@ -32,6 +33,7 @@ import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import kotlinx.coroutines.withTimeout
+import kotlinx.serialization.json.decodeFromJsonElement
 import java.util.concurrent.ConcurrentHashMap
 
 private const val TAG = "ChatViewModel"
@@ -845,7 +847,9 @@ class ChatViewModel(
                     val body = result.data
                     val platformsStr =
                         body.gateway_platforms?.entries?.joinToString("\n") { (k, v) ->
-                            "  • **$k**: ${v.state ?: "Unknown"}${if (v.error_code != null) " (Error: ${v.error_code})" else ""}"
+                            val status = v?.state ?: "Unknown"
+                            val err = v?.error_code?.let { " (Error: $it)" } ?: ""
+                            "  • **$k**: $status$err"
                         } ?: "No active platforms"
                     addAssistantMessage(
                         """
@@ -983,13 +987,15 @@ class ChatViewModel(
 
     fun refreshCurrentSession() {
         val sessionId = _uiState.value.currentSessionId ?: return
-        loadCachedMessages(sessionId)
-        // Skip the REST call if we already have messages — the WebSocket keeps
-        // the UI current. The REST endpoint can 404 on tab switch-back when the
-        // WS session ID doesn't round-trip through the resolver (issue #366),
-        // showing a misleading error banner while the connection is fine.
-        if (_uiState.value.messages.isNotEmpty()) return
-        loadSessionMessages(sessionId)
+        viewModelScope.launch {
+            loadCachedMessages(sessionId).join()
+            // Skip the REST call if we already have messages — the WebSocket keeps
+            // the UI current. The REST endpoint can 404 on tab switch-back when the
+            // WS session ID doesn't round-trip through the resolver (issue #366),
+            // showing a misleading error banner while the connection is fine.
+            if (_uiState.value.messages.isNotEmpty()) return@launch
+            loadSessionMessages(sessionId)
+        }
     }
 
     fun refreshSettings() {
@@ -1004,8 +1010,8 @@ class ChatViewModel(
     @Suppress("UNCHECKED_CAST")
     private fun parseCommandCatalog(map: Map<*, *>): CommandCatalog? =
         try {
-            val json = OkHttpProvider.gson.toJson(map)
-            OkHttpProvider.gson.fromJson(json, CommandCatalog::class.java)
+            val jsonElement = map.toJsonElement()
+            OkHttpProvider.json.decodeFromJsonElement<CommandCatalog>(jsonElement)
         } catch (e: Exception) {
             Log.e(TAG, "Failed to parse command catalog", e)
             null
@@ -1030,23 +1036,25 @@ class ChatViewModel(
             )
         }
         _streamingState.update { StreamingState() }
-        // Step 1: Load cached messages first — instant display
-        loadCachedMessages(sessionId)
-        // Step 2: Resume session on server
-        viewModelScope.launch(Dispatchers.IO) {
-            wsClient.send(
-                WsMethods.SESSION_RESUME,
-                mapOf("session_id" to sessionId),
-                onSent = { id -> trackRequest(id, WsMethods.SESSION_RESUME) },
-            )
+        viewModelScope.launch {
+            // Step 1: Load cached messages first — instant display
+            loadCachedMessages(sessionId).join()
+            // Step 2: Resume session on server
+            launch(Dispatchers.IO) {
+                wsClient.send(
+                    WsMethods.SESSION_RESUME,
+                    mapOf("session_id" to sessionId),
+                    onSent = { id -> trackRequest(id, WsMethods.SESSION_RESUME) },
+                )
+            }
+            // Step 3: Load fresh messages from REST and merge
+            loadSessionMessages(sessionId)
+            // Step 4: Refresh session list to get latest titles
+            loadSessions()
         }
-        // Step 3: Load fresh messages from REST and merge
-        loadSessionMessages(sessionId)
-        // Step 4: Refresh session list to get latest titles
-        loadSessions()
     }
 
-    private fun loadCachedMessages(sessionId: String) {
+    private fun loadCachedMessages(sessionId: String): Job =
         viewModelScope.launch(Dispatchers.IO) {
             val cachedMessages = repo.loadMessages(sessionId)
             _uiState.update { state ->
@@ -1058,7 +1066,6 @@ class ChatViewModel(
                 }
             }
         }
-    }
 
     private fun loadSessionMessages(sessionId: String) {
         viewModelScope.launch {
@@ -1082,7 +1089,7 @@ class ChatViewModel(
                             // Room duplicates when re-loading the same session.
                             val stableId = "rest-$sessionId-$index"
                             // Preserve original timestamp from the API when available
-                            val ts = msg.timestamp?.toLongOrNull() ?: System.currentTimeMillis()
+                            val ts = msg.timestampText?.toLongOrNull() ?: System.currentTimeMillis()
                             ChatMessage(
                                 id = stableId,
                                 role = role,

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatWsEventReducer.kt

@@ -2,6 +2,8 @@ package com.m57.hermescontrol.ui.chat
 
 import com.m57.hermescontrol.data.remote.OkHttpProvider
 import com.m57.hermescontrol.data.ws.WsEvent
+import com.m57.hermescontrol.data.ws.toJsonElement
+import kotlinx.serialization.encodeToString
 
 /**
  * Pure state reducer for WebSocket events.
@@ -225,7 +227,7 @@ object ChatWsEventReducer {
     ): ReducerResult {
         val contentJson =
             event.data?.let {
-                OkHttpProvider.gson.toJson(it)
+                OkHttpProvider.json.encodeToString(it.toJsonElement())
             } ?: ""
         val toolMessage =
             ChatMessage(
@@ -271,7 +273,7 @@ object ChatWsEventReducer {
     ): ReducerResult {
         val contentJson =
             event.data?.let {
-                OkHttpProvider.gson.toJson(it)
+                OkHttpProvider.json.encodeToString(it.toJsonElement())
             } ?: ""
         val messages = state.messages.toMutableList()
         val toolIdx =