[Server] Reorganize server handlers and transport structure for better organization (#86)

* refactor: reorganize server handlers and transport structure

* feat: move the test files to corresponding directories

* fix: linter issues

* fix: correct PHPDoc formatting and streamline handler imports

* refactor: update import statement for ServerBuilder to Builder

src/Server/Transport/TransportInterface.php

@@ -0,0 +1,68 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Server\Transport;
+
+use Symfony\Component\Uid\Uuid;
+
+/**
+ * @author Christopher Hertel <mail@christopher-hertel.de>
+ * @author Kyrian Obikwelu <koshnawaza@gmail.com>
+ */
+interface TransportInterface
+{
+    /**
+     * Initializes the transport.
+     */
+    public function initialize(): void;
+
+    /**
+     * Registers a callback that will be invoked whenever the transport receives an incoming message.
+     *
+     * @param callable(string $message, ?Uuid $sessionId): void $listener The callback function to execute when the message occurs
+     */
+    public function onMessage(callable $listener): void;
+
+    /**
+     * Starts the transport's execution process.
+     *
+     * - For a blocking transport like STDIO, this method will run a continuous loop.
+     * - For a single-request transport like HTTP, this will process the request
+     *   and return a result (e.g., a PSR-7 Response) to be sent to the client.
+     *
+     * @return mixed the result of the transport's execution, if any
+     */
+    public function listen(): mixed;
+
+    /**
+     * Sends a raw JSON-RPC message string back to the client.
+     *
+     * @param string               $data    The JSON-RPC message string to send
+     * @param array<string, mixed> $context The context of the message
+     */
+    public function send(string $data, array $context): void;
+
+    /**
+     * Registers a callback that will be invoked when a session needs to be destroyed.
+     * This can happen when a client disconnects or explicitly ends their session.
+     *
+     * @param callable(Uuid $sessionId): void $listener The callback function to execute when destroying a session
+     */
+    public function onSessionEnd(callable $listener): void;
+
+    /**
+     * Closes the transport and cleans up any resources.
+     *
+     * This method should be called when the transport is no longer needed.
+     * It should clean up any resources and close any connections.
+     */
+    public function close(): void;
+}