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/test/java/com/m57/hermescontrol/data/remote/HermesApiServiceMockWebServerTest.kt

@@ -3,6 +3,7 @@ package com.m57.hermescontrol.data.remote
 import com.m57.hermescontrol.data.model.SessionMessage
 import com.m57.hermescontrol.data.model.StatusResponse
 import kotlinx.coroutines.runBlocking
+import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.mockwebserver.MockResponse
 import okhttp3.mockwebserver.MockWebServer
 import org.junit.After
@@ -13,7 +14,7 @@ import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Test
 import retrofit2.Retrofit
-import retrofit2.converter.gson.GsonConverterFactory
+import retrofit2.converter.kotlinx.serialization.asConverterFactory
 
 /**
  * MockWebServer-based tests for [HermesApiService].
@@ -35,7 +36,7 @@ class HermesApiServiceMockWebServerTest {
             Retrofit
                 .Builder()
                 .baseUrl(mockServer.url("/"))
-                .addConverterFactory(GsonConverterFactory.create(OkHttpProvider.gson))
+                .addConverterFactory(OkHttpProvider.json.asConverterFactory("application/json".toMediaType()))
                 .build()
                 .create(HermesApiService::class.java)
     }
@@ -244,7 +245,7 @@ class HermesApiServiceMockWebServerTest {
 
             assertEquals("user", messages[0].role)
             assertEquals("Hello Hermes", messages[0].content)
-            assertEquals("1718000000", messages[0].timestamp)
+            assertEquals("1718000000", messages[0].timestampText)
 
             assertEquals("assistant", messages[1].role)
             assertEquals("Hi! How can I help?", messages[1].content)

app/src/test/java/com/m57/hermescontrol/data/remote/HermesApiServiceTest.kt

@@ -2,6 +2,7 @@ package com.m57.hermescontrol.data.remote
 
 import com.m57.hermescontrol.data.model.ToggleSkillRequest
 import kotlinx.coroutines.test.runTest
+import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.mockwebserver.MockResponse
 import okhttp3.mockwebserver.MockWebServer
 import org.junit.jupiter.api.AfterEach
@@ -13,7 +14,7 @@ import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
 import retrofit2.Retrofit
-import retrofit2.converter.gson.GsonConverterFactory
+import retrofit2.converter.kotlinx.serialization.asConverterFactory
 
 class HermesApiServiceTest {
     private lateinit var mockWebServer: MockWebServer
@@ -28,7 +29,7 @@ class HermesApiServiceTest {
             Retrofit
                 .Builder()
                 .baseUrl(mockWebServer.url("/"))
-                .addConverterFactory(GsonConverterFactory.create())
+                .addConverterFactory(OkHttpProvider.json.asConverterFactory("application/json".toMediaType()))
                 .build()
 
         apiService = retrofit.create(HermesApiService::class.java)
@@ -151,58 +152,49 @@ class HermesApiServiceTest {
         }
 
     @Test
-    fun testGetSkills_missingFieldsInResponse() =
-        runTest {
-            mockWebServer.enqueue(
-                MockResponse()
-                    .setResponseCode(200)
-                    .setBody("""[{"description":"info"}]"""),
-            )
+    fun testGetSkills_missingFieldsInResponse() {
+        mockWebServer.enqueue(
+            MockResponse()
+                .setResponseCode(200)
+                .setBody("""[{"description":"info"}]"""),
+        )
 
-            val response = apiService.getSkills()
-            assertTrue(response.isSuccessful)
-            val skills = response.body()
-            assertEquals(1, skills?.size)
-            val skill = skills?.get(0)
-            org.junit.jupiter.api.Assertions
-                .assertNull(skill?.name)
-            assertEquals(false, skill?.enabled)
+        assertThrows(kotlinx.serialization.SerializationException::class.java) {
+            runTest {
+                apiService.getSkills()
+            }
         }
+    }
 
     @Test
-    fun testGetCronJobs_missingFieldsInResponse() =
-        runTest {
-            mockWebServer.enqueue(
-                MockResponse()
-                    .setResponseCode(200)
-                    .setBody("""[{"schedule":"* * * * *"}]"""),
-            )
+    fun testGetCronJobs_missingFieldsInResponse() {
+        mockWebServer.enqueue(
+            MockResponse()
+                .setResponseCode(200)
+                .setBody("""[{"schedule":"* * * * *"}]"""),
+        )
 
-            val response = apiService.getCronJobs()
-            assertTrue(response.isSuccessful)
-            val jobs = response.body()
-            assertEquals(1, jobs?.size)
-            val job = jobs?.get(0)
-            org.junit.jupiter.api.Assertions
-                .assertNull(job?.id)
-            org.junit.jupiter.api.Assertions
-                .assertNull(job?.name)
+        assertThrows(kotlinx.serialization.SerializationException::class.java) {
+            runTest {
+                apiService.getCronJobs()
+            }
         }
+    }
 
     @Test
-    fun testGetSessions_missingSessionsField() =
-        runTest {
-            mockWebServer.enqueue(
-                MockResponse()
-                    .setResponseCode(200)
-                    .setBody("""{}"""),
-            )
+    fun testGetSessions_missingSessionsField() {
+        mockWebServer.enqueue(
+            MockResponse()
+                .setResponseCode(200)
+                .setBody("""{}"""),
+        )
 
-            val response = apiService.getSessions()
-            assertTrue(response.isSuccessful)
-            org.junit.jupiter.api.Assertions
-                .assertNull(response.body()?.sessions)
+        assertThrows(kotlinx.serialization.SerializationException::class.java) {
+            runTest {
+                apiService.getSessions()
+            }
         }
+    }
 
     @Test
     fun testGetSessionMessages_apiReturnsMalformedJson_throwsException() {
@@ -212,7 +204,7 @@ class HermesApiServiceTest {
                 .setBody("""{"messages": ["""),
         )
 
-        assertThrows(java.io.EOFException::class.java) {
+        assertThrows(kotlinx.serialization.SerializationException::class.java) {
             runTest {
                 apiService.getSessionMessages("session_1")
             }
@@ -220,19 +212,19 @@ class HermesApiServiceTest {
     }
 
     @Test
-    fun testGetSessionMessages_missingMessagesField() =
-        runTest {
-            mockWebServer.enqueue(
-                MockResponse()
-                    .setResponseCode(200)
-                    .setBody("""{}"""),
-            )
+    fun testGetSessionMessages_missingMessagesField() {
+        mockWebServer.enqueue(
+            MockResponse()
+                .setResponseCode(200)
+                .setBody("""{}"""),
+        )
 
-            val response = apiService.getSessionMessages("session_1")
-            assertTrue(response.isSuccessful)
-            org.junit.jupiter.api.Assertions
-                .assertNull(response.body()?.messages)
+        assertThrows(kotlinx.serialization.SerializationException::class.java) {
+            runTest {
+                apiService.getSessionMessages("session_1")
+            }
         }
+    }
 
     @Test
     fun testGetSkills_malformedJsonResponse_throwsException() =
@@ -243,7 +235,7 @@ class HermesApiServiceTest {
                     .setBody("""[{"name":"weather""""),
             )
 
-            org.junit.jupiter.api.Assertions.assertThrows(java.io.EOFException::class.java) {
+            org.junit.jupiter.api.Assertions.assertThrows(kotlinx.serialization.SerializationException::class.java) {
                 kotlinx.coroutines.runBlocking {
                     apiService.getSkills()
                 }
@@ -259,7 +251,7 @@ class HermesApiServiceTest {
                     .setBody("""[{"name":"weather","enabled":{}}]"""),
             )
 
-            org.junit.jupiter.api.Assertions.assertThrows(com.google.gson.JsonSyntaxException::class.java) {
+            org.junit.jupiter.api.Assertions.assertThrows(kotlinx.serialization.SerializationException::class.java) {
                 kotlinx.coroutines.runBlocking {
                     apiService.getSkills()
                 }