Add elicitation support for server-to-client user input requests (#228)

- Add ElicitAction enum (accept/decline/cancel)
- Add schema definitions for elicitation fields (string, number, boolean, enum)
- Add ElicitationSchema wrapper for requestedSchema
- Add ElicitRequest and ElicitResult for elicitation/create method
- Update ClientCapabilities to support elicitation capability
- Add elicit() method to ClientGateway
- Store client capabilities in session during initialization
- Add comprehensive unit tests
- Add elicitation example server with book_restaurant, confirm_action, and collect_feedback tools

examples/server/elicitation/server.php

@@ -0,0 +1,40 @@
+#!/usr/bin/env php
+<?php
+
+declare(strict_types=1);
+
+/*
+ * 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.
+ */
+
+/**
+ * MCP Elicitation Example Server.
+ *
+ * Demonstrates server-to-client elicitation for interactive user input during tool execution.
+ * See docs/examples.md for detailed documentation and usage examples.
+ */
+
+require_once dirname(__DIR__).'/bootstrap.php';
+chdir(__DIR__);
+
+use Mcp\Schema\ServerCapabilities;
+use Mcp\Server;
+use Mcp\Server\Session\FileSessionStore;
+
+$server = Server::builder()
+    ->setServerInfo('Elicitation Demo', '1.0.0')
+    ->setLogger(logger())
+    ->setContainer(container())
+    ->setSession(new FileSessionStore(__DIR__.'/sessions'))
+    ->setCapabilities(new ServerCapabilities(logging: true, tools: true))
+    ->setDiscovery(__DIR__)
+    ->build();
+
+$result = $server->run(transport());
+
+shutdown($result);