[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.

src/Server/Handler/Request/RequestHandlerInterface.php

@@ -17,7 +17,11 @@ use Mcp\Schema\JsonRpc\Response;
 use Mcp\Server\Session\SessionInterface;
 
 /**
- * @template TResult
+ * Covariant in TResult: a handler only ever produces its result, so one
+ * declaring a concrete result type satisfies a collection of handlers typed by
+ * the interface every result implements.
+ *
+ * @template-covariant TResult
  *
  * @author Kyrian Obikwelu <koshnawaza@gmail.com>
  */