chore(jetbrains): bump kotlinx-serialization to 1.8.1 (latest for Kotlin 2.1.x)

openapi-generator is already at the latest version (7.21.0).
kotlinx-serialization 1.9.0+ requires Kotlin 2.2.0, so 1.8.1 is the
correct ceiling for our Kotlin 2.1.20 toolchain.

packages/kilo-jetbrains/backend/src/test/kotlin/ai/kilocode/backend/KiloAppStateTest.kt

@@ -54,7 +54,7 @@ class KiloAppStateTest {
 
     @Test
     fun `AppData construction`() {
-        val cfg = Config()
+        val cfg = Config(model = "test")
         val data = AppData(profile = null, config = cfg, notifications = emptyList())
         assertNull(data.profile)
         assertEquals(cfg, data.config)

packages/kilo-jetbrains/gradle/libs.versions.toml

@@ -4,7 +4,7 @@ intellij-gradle-plugin = "2.10.5"
 intellij-rpc-plugin = "2.1.20-0.1"
 kotlin-jvm-plugin = "2.1.20"
 kotlin-serialization-plugin = "2.1.20"
-kotlin-serialization = "1.7.3"
+kotlin-serialization = "1.8.1"
 okhttp = "4.12.0"
 openapi-generator = "7.21.0"