[Schema][Server] Add the extensions framework SEP-2133 defines (#443)

* [Schema][Server] Add the extensions framework SEP-2133 defines

enableExtension() took any string and advertised it, with no way for an
extension to add a method: MessageFactory could not decode one, so nothing
downstream ever saw it.

Identifiers are now checked against the _meta key naming rules through
ExtensionIdentifier - a prefix is mandatory, since an unprefixed name has no
owner, and the modelcontextprotocol/mcp second labels the specification
reserves are recognised. An extension implementing
MethodProvidingExtensionInterface contributes both its message classes and
the handlers serving them.

MessageFactory::make() takes an $additional list of message classes, and
RequestHandlerInterface's result template is covariant so a handler
declaring a concrete result satisfies a collection typed by the interface.

* Fix stale @see reference in MethodProvidingExtensionInterface docblock

* [Schema][Server][Client] Make ExtensionIdentifier a value object

ExtensionInterface::getId() now returns ExtensionIdentifier, which
validates the SEP-2133 naming rules at construction instead of the
callers checking it. getMessages()/getRequestHandlers() move into
ExtensionInterface itself; AbstractExtension gives them empty
defaults for extensions that only announce a capability.

tests/Unit/Server/Extension/ThingListHandler.php

@@ -0,0 +1,33 @@
+<?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\Tests\Unit\Server\Extension;
+
+use Mcp\Schema\JsonRpc\Request;
+use Mcp\Schema\JsonRpc\Response;
+use Mcp\Server\Handler\Request\RequestHandlerInterface;
+use Mcp\Server\Session\SessionInterface;
+
+/**
+ * @implements RequestHandlerInterface<ThingListResult>
+ */
+final class ThingListHandler implements RequestHandlerInterface
+{
+    public function supports(Request $request): bool
+    {
+        return $request instanceof ThingListRequest;
+    }
+
+    public function handle(Request $request, SessionInterface $session): Response
+    {
+        return new Response($request->getId(), new ThingListResult(['a', 'b']));
+    }
+}