Add elicitation support for server-to-client user input requests (#228)

- Add ElicitAction enum (accept/decline/cancel)
- Add schema definitions for elicitation fields (string, number, boolean, enum)
- Add ElicitationSchema wrapper for requestedSchema
- Add ElicitRequest and ElicitResult for elicitation/create method
- Update ClientCapabilities to support elicitation capability
- Add elicit() method to ClientGateway
- Store client capabilities in session during initialization
- Add comprehensive unit tests
- Add elicitation example server with book_restaurant, confirm_action, and collect_feedback tools

src/Server/Handler/Request/InitializeHandler.php

@@ -45,6 +45,7 @@ final class InitializeHandler implements RequestHandlerInterface
         \assert($request instanceof InitializeRequest);
 
         $session->set('client_info', $request->clientInfo->jsonSerialize());
+        $session->set('client_capabilities', $request->capabilities->jsonSerialize());
 
         return new Response(
             $request->getId(),