[Server] Add Bidirectional Client Communication Support (#109)

* feat(server): add bidirectional client communication support

* fix(server): use correct session ID string conversion method

* fix: clean up docblocks for request handler methods in Builder

* fix: merge artifacts

examples/custom-method-handlers/CallToolRequestHandler.php

@@ -0,0 +1,73 @@
+<?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\Example\CustomMethodHandlers;
+
+use Mcp\Schema\Content\TextContent;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Request\CallToolRequest;
+use Mcp\Schema\Result\CallToolResult;
+use Mcp\Schema\Tool;
+use Mcp\Server\Handler\Request\RequestHandlerInterface;
+use Mcp\Server\Session\SessionInterface;
+
+/** @implements RequestHandlerInterface<CallToolResult> */
+class CallToolRequestHandler implements RequestHandlerInterface
+{
+    /**
+     * @param array<string, Tool> $toolDefinitions
+     */
+    public function __construct(private array $toolDefinitions)
+    {
+    }
+
+    public function supports(Request $request): bool
+    {
+        return $request instanceof CallToolRequest;
+    }
+
+    /**
+     * @return Response<CallToolResult>|Error
+     */
+    public function handle(Request $request, SessionInterface $session): Response|Error
+    {
+        \assert($request instanceof CallToolRequest);
+
+        $name = $request->name;
+        $args = $request->arguments ?? [];
+
+        if (!isset($this->toolDefinitions[$name])) {
+            return new Error($request->getId(), Error::METHOD_NOT_FOUND, \sprintf('Tool not found: %s', $name));
+        }
+
+        try {
+            switch ($name) {
+                case 'say_hello':
+                    $greetName = (string) ($args['name'] ?? 'world');
+                    $result = [new TextContent(\sprintf('Hello, %s!', $greetName))];
+                    break;
+                case 'sum':
+                    $a = (float) ($args['a'] ?? 0);
+                    $b = (float) ($args['b'] ?? 0);
+                    $result = [new TextContent((string) ($a + $b))];
+                    break;
+                default:
+                    $result = [new TextContent('Unknown tool')];
+            }
+
+            return new Response($request->getId(), new CallToolResult($result));
+        } catch (\Throwable $e) {
+            return new Response($request->getId(), new CallToolResult([new TextContent('Tool execution failed')], true));
+        }
+    }
+}