Introduce server sessions (#198)

Introduce a `ServerSession` to manage multiple connections to the mcp
server. `Protocall` logic of the `Server` was moved to `ServerSession`,
while MCP resources logic remained in `Server`

## Motivation and Context
1. Fix the multiple connection issues reported for SSE transport
2. Avoid server duplications during configurations: currently every connection
creates a session, but the server stays single

## How Has This Been Tested?

Added `SseIntegrationTest` and `WebSocketIntegrationTest` integration
tests to check connection with single and multiple clients

## Breaking Changes

This change caused several depreciations:
1. The `Server` -> `Protocol` inheritance (which caused single
connection issue) should be removed in future releases, now, all the
methods inherited from `Protocall` marked as `Deprecated` with a
`Warning`
2. Websocket Ktor server initialization changes in order not to create
server instance per connection

## Types of changes

- [ ] 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
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

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

---------

Co-authored-by: devcrocod <devcrocod@gmail.com>

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/Client.kt

@@ -157,9 +157,11 @@ public open class Client(private val clientInfo: Implementation, options: Client
 
             notification(InitializedNotification())
         } catch (error: Throwable) {
+            logger.error(error) { "Failed to initialize client: ${error.message}" }
             close()
+
             if (error !is CancellationException) {
-                throw IllegalStateException("Error connecting to transport: ${error.message}")
+                throw IllegalStateException("Error connecting to transport: ${error.message}", error)
             }
 
             throw error

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SSEClientTransport.kt

@@ -1,5 +1,6 @@
 package io.modelcontextprotocol.kotlin.sdk.client
 
+import io.github.oshai.kotlinlogging.KotlinLogging
 import io.ktor.client.HttpClient
 import io.ktor.client.plugins.sse.ClientSSESession
 import io.ktor.client.plugins.sse.sseSession
@@ -46,6 +47,8 @@ public class SseClientTransport(
     private val reconnectionTime: Duration? = null,
     private val requestBuilder: HttpRequestBuilder.() -> Unit = {},
 ) : AbstractTransport() {
+    private val logger = KotlinLogging.logger {}
+
     private val initialized: AtomicBoolean = AtomicBoolean(false)
     private val endpoint = CompletableDeferred<String>()
 
@@ -111,6 +114,8 @@ public class SseClientTransport(
                 val text = response.bodyAsText()
                 error("Error POSTing to endpoint (HTTP ${response.status}): $text")
             }
+
+            logger.debug { "Client successfully sent message via SSE $endpoint" }
         } catch (e: Throwable) {
             _onError(e)
             throw e
@@ -158,6 +163,7 @@ public class SseClientTransport(
             val path = if (eventData.startsWith("/")) eventData.substring(1) else eventData
             val endpointUrl = Url("$baseUrl/$path")
             endpoint.complete(endpointUrl.toString())
+            logger.debug { "Client connected to endpoint: $endpointUrl" }
         } catch (e: Throwable) {
             _onError(e)
             endpoint.completeExceptionally(e)

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/WebSocketClientTransport.kt

@@ -1,5 +1,6 @@
 package io.modelcontextprotocol.kotlin.sdk.client
 
+import io.github.oshai.kotlinlogging.KotlinLogging
 import io.ktor.client.HttpClient
 import io.ktor.client.plugins.websocket.webSocketSession
 import io.ktor.client.request.HttpRequestBuilder
@@ -10,6 +11,8 @@ import io.modelcontextprotocol.kotlin.sdk.shared.MCP_SUBPROTOCOL
 import io.modelcontextprotocol.kotlin.sdk.shared.WebSocketMcpTransport
 import kotlin.properties.Delegates
 
+private val logger = KotlinLogging.logger {}
+
 /**
  * Client transport for WebSocket: this will connect to a server over the WebSocket protocol.
  */
@@ -21,6 +24,8 @@ public class WebSocketClientTransport(
     override var session: WebSocketSession by Delegates.notNull()
 
     override suspend fun initializeSession() {
+        logger.debug { "Websocket session initialization started..." }
+
         session = urlString?.let {
             client.webSocketSession(it) {
                 requestBuilder()
@@ -32,5 +37,7 @@ public class WebSocketClientTransport(
 
             header(HttpHeaders.SecWebSocketProtocol, MCP_SUBPROTOCOL)
         }
+
+        logger.debug { "Websocket session initialization finished" }
     }
 }

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/WebSocketMcpKtorClientExtensions.kt

@@ -1,11 +1,14 @@
 package io.modelcontextprotocol.kotlin.sdk.client
 
+import io.github.oshai.kotlinlogging.KotlinLogging
 import io.ktor.client.HttpClient
 import io.ktor.client.request.HttpRequestBuilder
 import io.modelcontextprotocol.kotlin.sdk.Implementation
 import io.modelcontextprotocol.kotlin.sdk.LIB_VERSION
 import io.modelcontextprotocol.kotlin.sdk.shared.IMPLEMENTATION_NAME
 
+private val logger = KotlinLogging.logger {}
+
 /**
  * Returns a new WebSocket transport for the Model Context Protocol using the provided HttpClient.
  *
@@ -36,6 +39,8 @@ public suspend fun HttpClient.mcpWebSocket(
             version = LIB_VERSION,
         ),
     )
+    logger.debug { "Client started to connect to server" }
     client.connect(transport)
+    logger.debug { "Client finished to connect to server" }
     return client
 }