A nullable tool parameter with a default ignores an explicit null argument #640

Description

ReferenceHandler::prepareArguments() tests isset($arguments[$paramName]), so an argument the client explicitly sends as null is treated as missing: a parameter like ?string $category = 'books' receives 'books', not null. The schema allows null, the client sent null, the handler runs with another value.

How to reproduce

final class Shop
{
    public function search(?string $category = 'books'): string
    {
        return var_export($category, true);
    }
}

// Server::builder()->setServerInfo('repro', '1.0.0')->addTool([Shop::class, 'search'], 'search')->build()->run($transport);
// tools/call {"name":"search","arguments":{"category":null}}

Actual: "text":"'books'" Expected: "text":"NULL"

Possible solution

Use array_key_exists($paramName, $arguments) in src/Capability/Registry/ReferenceHandler.php (line 145 at 5d311d6); castArgumentType() already returns null for a nullable type.

Versions: mcp/sdk main (5d311d6) and v0.9.0, PHP 8.3.