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

tests/Unit/Schema/Request/ElicitRequestTest.php

@@ -0,0 +1,74 @@
+<?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.
+ */
+
+namespace Mcp\Tests\Unit\Schema\Request;
+
+use Mcp\Schema\Elicitation\ElicitationSchema;
+use Mcp\Schema\Elicitation\NumberSchemaDefinition;
+use Mcp\Schema\Elicitation\StringSchemaDefinition;
+use Mcp\Schema\Request\ElicitRequest;
+use PHPUnit\Framework\TestCase;
+
+final class ElicitRequestTest extends TestCase
+{
+    public function testConstructor(): void
+    {
+        $schema = new ElicitationSchema([
+            'name' => new StringSchemaDefinition('Name'),
+        ]);
+
+        $request = new ElicitRequest('Please provide your name', $schema);
+
+        $this->assertSame('Please provide your name', $request->message);
+        $this->assertSame($schema, $request->requestedSchema);
+    }
+
+    public function testGetMethod(): void
+    {
+        $this->assertSame('elicitation/create', ElicitRequest::getMethod());
+    }
+
+    public function testJsonSerialization(): void
+    {
+        $schema = new ElicitationSchema(
+            [
+                'name' => new StringSchemaDefinition('Name'),
+                'age' => new NumberSchemaDefinition('Age', integerOnly: true, minimum: 0),
+            ],
+            ['name'],
+        );
+
+        $request = new ElicitRequest('Please provide your details', $schema);
+        $request = $request->withId(1);
+
+        $json = json_encode($request);
+        $this->assertIsString($json);
+
+        $decoded = json_decode($json, true);
+        $this->assertIsArray($decoded);
+
+        $this->assertSame('2.0', $decoded['jsonrpc']);
+        $this->assertSame('elicitation/create', $decoded['method']);
+        $this->assertArrayHasKey('params', $decoded);
+
+        $params = $decoded['params'];
+        $this->assertSame('Please provide your details', $params['message']);
+        $this->assertArrayHasKey('requestedSchema', $params);
+
+        $requestedSchema = $params['requestedSchema'];
+        $this->assertSame('object', $requestedSchema['type']);
+        $this->assertArrayHasKey('name', $requestedSchema['properties']);
+        $this->assertArrayHasKey('age', $requestedSchema['properties']);
+        $this->assertSame(['name'], $requestedSchema['required']);
+    }
+}