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

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/AbstractResourceIntegrationTest.kt

@@ -19,7 +19,6 @@ import kotlinx.coroutines.test.runTest
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
 import java.util.concurrent.atomic.AtomicBoolean
-import kotlin.test.Ignore
 import kotlin.test.assertEquals
 import kotlin.test.assertNotNull
 import kotlin.test.assertTrue
@@ -165,7 +164,6 @@ abstract class AbstractResourceIntegrationTest : KotlinTestBase() {
         assertEquals(testResourceContent, content.text, "Resource content should match")
     }
 
-    @Ignore("Blocked by https://github.com/modelcontextprotocol/kotlin-sdk/issues/249")
     @Test
     fun testSubscribeAndUnsubscribe() {
         runBlocking(Dispatchers.IO) {

integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/KotlinTestBase.kt

@@ -148,7 +148,9 @@ abstract class KotlinTestBase {
                 // Create StreamableHTTP server transport
                 // Using JSON response mode for simpler testing (no SSE session required)
                 val transport = StreamableHttpServerTransport(
-                    enableJsonResponse = true, // Use JSON response mode for testing
+                    StreamableHttpServerTransport.Configuration(
+                        enableJsonResponse = true, // Use JSON response mode for testing
+                    ),
                 )
                 // Use stateless mode to skip session validation for simpler testing
                 transport.setSessionIdGenerator(null)