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/config/ConfigScreen.kt

@@ -53,8 +53,6 @@ import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.lifecycle.viewmodel.compose.viewModel
-import com.google.gson.JsonElement
-import com.google.gson.JsonPrimitive
 import com.m57.hermescontrol.R
 import com.m57.hermescontrol.data.model.SchemaField
 import com.m57.hermescontrol.ui.common.ErrorState
@@ -63,6 +61,11 @@ import com.m57.hermescontrol.ui.common.LoadingState
 import com.m57.hermescontrol.ui.common.NavIcon
 import com.m57.hermescontrol.ui.common.SearchBar
 import com.m57.hermescontrol.ui.common.ToastEffect
+import kotlinx.serialization.json.JsonArray
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.booleanOrNull
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -580,7 +583,7 @@ private fun ConfigField(
                             modifier = Modifier.weight(1f),
                         )
                         Switch(
-                            checked = currentValue?.asBoolean ?: false,
+                            checked = (currentValue as? JsonPrimitive)?.booleanOrNull ?: false,
                             onCheckedChange = { onChange(JsonPrimitive(it)) },
                         )
                     }
@@ -590,7 +593,7 @@ private fun ConfigField(
                     DropdownField(
                         label = label,
                         options = field.options ?: emptyList(),
-                        selectedValue = currentValue?.asString ?: "",
+                        selectedValue = (currentValue as? JsonPrimitive)?.content ?: "",
                         onOptionSelected = { onChange(JsonPrimitive(it)) },
                     )
                     if (description != label) {
@@ -607,7 +610,7 @@ private fun ConfigField(
                     OutlinedTextField(
                         value =
                             currentValue?.let {
-                                if (it.isJsonPrimitive) it.asString else ""
+                                if (it is JsonPrimitive) it.content else ""
                             } ?: "",
                         onValueChange = { text ->
                             text.toDoubleOrNull()?.let { doubleVal ->
@@ -630,10 +633,10 @@ private fun ConfigField(
                     // String or other type
                     val textValue =
                         currentValue?.let {
-                            when {
-                                it.isJsonPrimitive -> it.asString
-                                it.isJsonObject -> it.toString()
-                                it.isJsonArray -> it.toString()
+                            when (it) {
+                                is JsonPrimitive -> it.content
+                                is JsonObject -> it.toString()
+                                is JsonArray -> it.toString()
                                 else -> ""
                             }
                         } ?: ""
@@ -708,20 +711,17 @@ private fun collectUncoveredPaths(
         val dotPath = if (prefix.isEmpty()) key else "$prefix.$key"
         // If this exact path is in the schema, it's covered — skip entirely
         if (dotPath in schemaPaths) continue
-        if (value is com.google.gson.JsonObject) {
-            // Recurse into nested objects to find uncovered leaves
-            val subKeys = value.keySet()
+        if (value is JsonObject) {
             // Before recursing deeper, check if this whole sub-tree has any schema coverage
             val hasSchemaCoverage = schemaPaths.any { it == dotPath || it.startsWith("$dotPath.") }
             if (hasSchemaCoverage) {
                 // Schema covers some sub-paths — recurse to find what's NOT covered
-                val subMap = subKeys.associateWith { value.get(it) }
-                result.addAll(collectUncoveredPaths(subMap, schemaPaths, dotPath))
+                result.addAll(collectUncoveredPaths(value, schemaPaths, dotPath))
             } else {
                 // No schema coverage at all for this subtree — add the whole thing
                 result.add(dotPath)
             }
-        } else if (value is com.google.gson.JsonArray) {
+        } else if (value is JsonArray) {
             // Arrays: check if the path is in schema; if not, add it
             if (dotPath !in schemaPaths) {
                 result.add(dotPath)
@@ -746,7 +746,7 @@ private fun getJsonValue(
         if (i == 0) {
             current = config[key]
         } else {
-            current = current?.asJsonObject?.get(key)
+            current = (current as? JsonObject)?.get(key)
         }
         if (current == null) return null
     }

app/src/main/java/com/m57/hermescontrol/ui/config/ConfigViewModel.kt

@@ -2,8 +2,6 @@ package com.m57.hermescontrol.ui.config
 
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
-import com.google.gson.JsonElement
-import com.google.gson.JsonObject
 import com.m57.hermescontrol.data.model.ConfigSchemaResponse
 import com.m57.hermescontrol.data.model.ConfigUpdateRequest
 import com.m57.hermescontrol.data.model.UpdateRawConfigRequest
@@ -19,6 +17,8 @@ import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.update
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
+import kotlinx.serialization.json.JsonElement
+import kotlinx.serialization.json.JsonObject
 
 data class ConfigUiState(
     val isLoading: Boolean = false,
@@ -296,15 +296,14 @@ class ConfigViewModel :
         val topKey = parts.first()
         val rest = parts.drop(1).joinToString(".")
         val topValue = config[topKey]
-        val nestedJson = topValue?.asJsonObject ?: JsonObject()
+        val nestedJson = (topValue as? JsonObject) ?: JsonObject(emptyMap())
         val updatedNested =
             applyNestedValue(
-                nestedJson.entrySet().associate { it.key to it.value },
+                nestedJson,
                 rest,
                 value,
             )
-        val nestedObj = JsonObject()
-        updatedNested.forEach { (k, v) -> nestedObj.add(k, v) }
+        val nestedObj = JsonObject(updatedNested)
         val mutable = config.toMutableMap()
         mutable[topKey] = nestedObj
         return mutable
@@ -312,23 +311,38 @@ class ConfigViewModel :
 
     /** Build a nested JSON changeset from dot-path pending changes. */
     private fun buildChangeset(changes: Map<String, JsonElement>): Map<String, JsonElement> {
-        val root = JsonObject()
+        val root = mutableMapOf<String, Any>()
         for ((dotPath, value) in changes) {
             val parts = dotPath.split(".")
             var current = root
             for (i in 0 until parts.size - 1) {
                 val key = parts[i]
-                val existing = current.get(key)
-                if (existing == null || !existing.isJsonObject) {
-                    val newObj = JsonObject()
-                    current.add(key, newObj)
-                    current = newObj
+                val existing = current[key]
+                if (existing !is MutableMap<*, *>) {
+                    val newMap = mutableMapOf<String, Any>()
+                    current[key] = newMap
+                    current = newMap
                 } else {
-                    current = existing.asJsonObject
+                    @Suppress("UNCHECKED_CAST")
+                    current = existing as MutableMap<String, Any>
                 }
             }
-            current.add(parts.last(), value)
+            current[parts.last()] = value
         }
-        return root.entrySet().associate { it.key to it.value as JsonElement }
+
+        fun toJsonObject(map: Map<String, Any>): JsonObject {
+            val content =
+                map.mapValues { (_, v) ->
+                    if (v is Map<*, *>) {
+                        @Suppress("UNCHECKED_CAST")
+                        toJsonObject(v as Map<String, Any>)
+                    } else {
+                        v as JsonElement
+                    }
+                }
+            return JsonObject(content)
+        }
+
+        return toJsonObject(root)
     }
 }