Ktor extensions for StreamableHttpServerTransport

Create Ktor extensions for StreamableHttpServerTransport.

Server should support both stateful and stateless operation mode.

1. Application-level extensions.

API might look like. Naming is an open question

application {
   routing {
     mcpStreamableHttp(config: StreamableHttpServerTransport.Configuration) { mcpServer }
   }
}

2. Route-level extensions. API might look like

application {
    install(SSE)
    routing {
        route("/mcp") {
            mcpStreamableHttp(config: StreamableHttpServerTransport.Configuration) { mcpServer }
        }
    }
}

Definition of Done:

For both (Application+Route level extensions) x (Statefull+stateless mode) = (4 combinations):

1. Routes implemented
2. Integration test for happy scenario
3. Unit tests for route initialization, like this

Related PRs:

• #504 initial implementation
• #560 streamable transport configuration
• #563
• #535

Start by reviewing the initial implementation in #504 and the transport configuration in #560, then compare the referenced KtorStatelessStreamableHttpRouteExtensionsTest. Define the application- and route-level extensions for both stateful and stateless modes, with routes, happy-path integration tests, and route-initialization unit tests covering all four combinations.