refactor(core): Fix Kotlin language and api version to 2.1 in build config (#620)

## Fix Kotlin language and api version to 2.1

- Remove language constructs not supported in Kotlin 2.1

<!-- Provide a brief summary of your changes -->

## Motivation and Context
To ensure binary compatibility

## How Has This Been Tested?
Regression tests

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [x] Build update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

buildSrc/src/main/kotlin/mcp.multiplatform.gradle.kts

@@ -5,6 +5,7 @@ import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 import org.jetbrains.kotlin.gradle.dsl.ExplicitApiMode
 import org.jetbrains.kotlin.gradle.dsl.JvmDefaultMode
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 
 plugins {
     kotlin("multiplatform")
@@ -15,6 +16,8 @@ plugins {
 kotlin {
 
     compilerOptions {
+        languageVersion = KotlinVersion.KOTLIN_2_1
+        apiVersion = KotlinVersion.KOTLIN_2_1
         freeCompilerArgs =
             listOf(
                 "-Wextra",

kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/streamable/http/StreamableHttpClientTransportTest.kt

@@ -148,18 +148,17 @@ class StreamableHttpClientTransportTest {
         )
 
         val transport = createTransport { request ->
-            when (
-                val msg = McpJson.decodeFromString<JSONRPCMessage>(
-                    (request.body as TextContent).text,
-                )
-            ) {
-                is JSONRPCRequest if msg.method == "initialize" -> respond(
+            val msg = McpJson.decodeFromString<JSONRPCMessage>(
+                (request.body as TextContent).text,
+            )
+            when {
+                msg is JSONRPCRequest && msg.method == "initialize" -> respond(
                     content = "",
                     status = HttpStatusCode.OK,
                     headers = headersOf("mcp-session-id", "test-session-id"),
                 )
 
-                is JSONRPCNotification if msg.method == "test" -> {
+                msg is JSONRPCNotification && msg.method == "test" -> {
                     assertEquals("test-session-id", request.headers["mcp-session-id"])
                     respond(
                         content = "",
@@ -246,8 +245,8 @@ class StreamableHttpClientTransportTest {
         var sseStarted = false
 
         val transport = createTransport { request ->
-            when (request.method) {
-                HttpMethod.Post if request.body.toString().contains("notifications/initialized") -> {
+            when {
+                request.method == HttpMethod.Post && request.body.toString().contains("notifications/initialized") -> {
                     respond(
                         content = "",
                         status = HttpStatusCode.Accepted,
@@ -256,7 +255,7 @@ class StreamableHttpClientTransportTest {
                 }
 
                 // Handle SSE connection
-                HttpMethod.Get -> {
+                request.method == HttpMethod.Get -> {
                     sseStarted = true
                     val sseContent = buildString {
                         // Server sends various notifications
@@ -288,7 +287,7 @@ class StreamableHttpClientTransportTest {
                 }
 
                 // Handle regular notifications
-                HttpMethod.Post -> {
+                request.method == HttpMethod.Post -> {
                     respond(
                         content = "",
                         status = HttpStatusCode.Accepted,

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/request.kt

@@ -16,9 +16,9 @@ import kotlin.jvm.JvmInline
 public value class RequestMeta(public val json: JsonObject) {
     public val progressToken: ProgressToken?
         get() = json["progressToken"]?.let { element ->
-            when (element) {
-                is JsonPrimitive if (element.isString) -> ProgressToken(element.content)
-                is JsonPrimitive if (element.longOrNull != null) -> ProgressToken(element.long)
+            when {
+                element is JsonPrimitive && element.isString -> ProgressToken(element.content)
+                element is JsonPrimitive && element.longOrNull != null -> ProgressToken(element.long)
                 else -> null
             }
         }

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/serializers.kt

@@ -414,9 +414,9 @@ internal object JSONRPCMessagePolymorphicSerializer :
  * Supports both string and number IDs.
  */
 internal object RequestIdPolymorphicSerializer : JsonContentPolymorphicSerializer<RequestId>(RequestId::class) {
-    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<RequestId> = when (element) {
-        is JsonPrimitive if (element.isString) -> RequestId.StringId.serializer()
-        is JsonPrimitive if (element.longOrNull != null) -> RequestId.NumberId.serializer()
+    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<RequestId> = when {
+        element is JsonPrimitive && element.isString -> RequestId.StringId.serializer()
+        element is JsonPrimitive && element.longOrNull != null -> RequestId.NumberId.serializer()
         else -> throw SerializationException("Invalid RequestId type: $element")
     }
 }