[Client] Add client-side elicitation support (#371)

* [Client] Add client-side elicitation support

The SDK could send elicitation/create requests from a server tool via
ClientGateway::elicit(), but the client could not receive them: there was
no request handler, and ElicitRequest was not registered in MessageFactory,
so incoming elicitation/create messages failed to parse.

This adds the client half, mirroring the existing Sampling pattern:

- Register ElicitRequest in MessageFactory so elicitation/create parses.
- ElicitationRequestHandler wraps a user callback and returns an
  ElicitResult (accept/decline/cancel) or an Error.
- ElicitationCallbackInterface defines the callback contract.
- ElicitationException forwards a specific message to the server; any
  other throwable returns a generic error.

Includes unit tests, a runnable client example against the elicitation
demo server, and client docs/README updates.

* [Client] Simplify elicitation example labelFor() per review

Type the parameter as AbstractSchemaDefinition and read ->title directly
instead of matching on each concrete definition type. title is a required
non-null string on the base class, so the match and the $name fallback were
redundant.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/Client/Handler/Request/ElicitationCallbackInterface.php

@@ -0,0 +1,26 @@
+<?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\Client\Handler\Request;
+
+use Mcp\Schema\Request\ElicitRequest;
+use Mcp\Schema\Result\ElicitResult;
+
+/**
+ * Contract for callbacks used by ElicitationRequestHandler.
+ *
+ * Implementations present the requested schema to the user and collect their
+ * response when the server sends an elicitation/create request.
+ */
+interface ElicitationCallbackInterface
+{
+    public function __invoke(ElicitRequest $request): ElicitResult;
+}

src/Client/Handler/Request/ElicitationRequestHandler.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\Client\Handler\Request;
+
+use Mcp\Exception\ElicitationException;
+use Mcp\Schema\JsonRpc\Error;
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Request\ElicitRequest;
+use Mcp\Schema\Result\ElicitResult;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+/**
+ * Handler for elicitation requests from the server.
+ *
+ * The MCP server may request additional information from the user during tool
+ * execution. This handler wraps a user-provided callback that presents the
+ * requested schema to the user and returns their response.
+ *
+ * @implements RequestHandlerInterface<ElicitResult>
+ *
+ * @author Johannes Wachter <johannes@sulu.io>
+ */
+class ElicitationRequestHandler implements RequestHandlerInterface
+{
+    public function __construct(
+        private readonly ElicitationCallbackInterface $callback,
+        private readonly LoggerInterface $logger = new NullLogger(),
+    ) {
+    }
+
+    public function supports(Request $request): bool
+    {
+        return $request instanceof ElicitRequest;
+    }
+
+    /**
+     * @return Response<ElicitResult>|Error
+     */
+    public function handle(Request $request): Response|Error
+    {
+        \assert($request instanceof ElicitRequest);
+
+        try {
+            $result = $this->callback->__invoke($request);
+
+            return new Response($request->getId(), $result);
+        } catch (ElicitationException $e) {
+            $this->logger->error('Elicitation failed: '.$e->getMessage(), ['exception' => $e]);
+
+            return Error::forInternalError($e->getMessage(), $request->getId());
+        } catch (\Throwable $e) {
+            $this->logger->error('Unexpected error during elicitation', ['exception' => $e]);
+
+            return Error::forInternalError('Error while processing elicitation', $request->getId());
+        }
+    }
+}