feat: introduce StreamableHttpServerTransport.Configuration (#560)

## Introduce Configuration class for StreamableHttpServerTransport

Replace the six-parameter flat constructor of
`StreamableHttpServerTransport` with a typed `Configuration` class. This
improves API ergonomics, enables structural equality and copy(), and
provides a stable extension point for future options without further
breaking the constructor signature.

### Changes:

- Add `Configuration` as a public class nested directly on
`StreamableHttpServerTransport`, with `enableJsonResponse` as the first
parameter (most commonly set)
  - Change the primary constructor to accept `Configuration`.
- Rename `retryIntervalMillis: Long?` to `retryInterval: Duration?` in
Configuration, aligning with Kotlin's type-safe time API
  - Deprecate the old flat constructor with a compatibility bridge
  - Update KotlinTestBase integration test to use the new constructor
- Enable test
AbstractResourceIntegrationTest.testSubscribeAndUnsubscribe() since #249
is closed

## Motivation and Context

The current StreamableHttpServerTransport API cannot be easily extended:
adding more parameters would be a breaking change. But this is already
needed for #521.
This PR is a prerequisite for #535

## How Has This Been Tested?
Regression tests

## Breaking Changes
No. Current StreamableHttpServerTransport constructor was deprecated

## 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)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation 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

kotlin-sdk-server/api/kotlin-sdk-server.api

@@ -195,6 +195,7 @@ public final class io/modelcontextprotocol/kotlin/sdk/server/StdioServerTranspor
 public final class io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport : io/modelcontextprotocol/kotlin/sdk/shared/AbstractTransport {
 	public static final field STANDALONE_SSE_STREAM_ID Ljava/lang/String;
 	public fun <init> ()V
+	public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport$Configuration;)V
 	public fun <init> (ZZLjava/util/List;Ljava/util/List;Lio/modelcontextprotocol/kotlin/sdk/server/EventStore;Ljava/lang/Long;)V
 	public synthetic fun <init> (ZZLjava/util/List;Ljava/util/List;Lio/modelcontextprotocol/kotlin/sdk/server/EventStore;Ljava/lang/Long;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public fun close (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
@@ -211,6 +212,17 @@ public final class io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServe
 	public fun start (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 }
 
+public final class io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport$Configuration {
+	public synthetic fun <init> (ZZLjava/util/List;Ljava/util/List;Lio/modelcontextprotocol/kotlin/sdk/server/EventStore;Lkotlin/time/Duration;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public synthetic fun <init> (ZZLjava/util/List;Ljava/util/List;Lio/modelcontextprotocol/kotlin/sdk/server/EventStore;Lkotlin/time/Duration;Lkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public final fun getAllowedHosts ()Ljava/util/List;
+	public final fun getAllowedOrigins ()Ljava/util/List;
+	public final fun getEnableDnsRebindingProtection ()Z
+	public final fun getEnableJsonResponse ()Z
+	public final fun getEventStore ()Lio/modelcontextprotocol/kotlin/sdk/server/EventStore;
+	public final fun getRetryInterval-FghU774 ()Lkotlin/time/Duration;
+}
+
 public final class io/modelcontextprotocol/kotlin/sdk/server/WebSocketMcpKtorServerExtensionsKt {
 	public static final fun mcpWebSocket (Lio/ktor/server/application/Application;Ljava/lang/String;Lkotlin/jvm/functions/Function0;)V
 	public static final fun mcpWebSocket (Lio/ktor/server/application/Application;Lkotlin/jvm/functions/Function0;)V