[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/Apps/McpAppsTest.php

@@ -27,7 +27,7 @@ class McpAppsTest extends TestCase
         $extension = new McpApps();
 
         $this->assertInstanceOf(ExtensionInterface::class, $extension);
-        $this->assertSame('io.modelcontextprotocol/ui', $extension->getId());
+        $this->assertSame('io.modelcontextprotocol/ui', (string) $extension->getId());
         $this->assertSame(['mimeTypes' => ['text/html;profile=mcp-app']], $extension->getCapabilities());
     }