fix: replace method_exists with instanceof for message ID extraction

Author: CodeWithKyrian

src/JsonRpc/Handler.php

@@ -23,6 +23,7 @@
 use Mcp\Schema\Implementation;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\HasMethodInterface;
+use Mcp\Schema\JsonRpc\Request;
 use Mcp\Schema\JsonRpc\Response;
 use Mcp\Schema\Request\InitializeRequest;
 use Mcp\Server\MethodHandlerInterface;
@@ -158,11 +159,9 @@ public function process(string $input, ?Uuid $sessionId): iterable
         }
 
         foreach ($messages as $message) {
-            $messageId = method_exists($message, 'getId') ? $message->getId() : 0;
-
             if ($message instanceof InvalidInputMessageException) {
                 $this->logger->warning('Failed to create message.', ['exception' => $message]);
-                $error = Error::forInvalidRequest($message->getMessage(), $messageId);
+                $error = Error::forInvalidRequest($message->getMessage());
                 yield [$this->encodeResponse($error), []];
                 continue;
             }
@@ -171,6 +170,8 @@ public function process(string $input, ?Uuid $sessionId): iterable
                 'method' => $message->getMethod(),
             ]);
 
+            $messageId = $message instanceof Request ? $message->getId() : 0;
+
             try {
                 $response = $this->handle($message, $session);
                 yield [$this->encodeResponse($response), ['session_id' => $session->getId()]];