[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/Schema/Extension/AbstractExtensionTest.php

@@ -0,0 +1,39 @@
+<?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\Schema\Extension;
+
+use Mcp\Schema\Extension\AbstractExtension;
+use Mcp\Schema\Extension\ExtensionIdentifier;
+use PHPUnit\Framework\Attributes\TestDox;
+use PHPUnit\Framework\TestCase;
+
+class AbstractExtensionTest extends TestCase
+{
+    #[TestDox('An extension that only announces a capability need not declare messages or handlers')]
+    public function testDefaultsToNoMessagesOrHandlers(): void
+    {
+        $extension = new class extends AbstractExtension {
+            public function getId(): ExtensionIdentifier
+            {
+                return new ExtensionIdentifier('com.example/minimal');
+            }
+
+            public function getCapabilities(): array
+            {
+                return [];
+            }
+        };
+
+        $this->assertSame([], $extension->getMessages());
+        $this->assertSame([], $extension->getRequestHandlers());
+    }
+}