class TestCommand extends Command
{
    // ...

    protected function execute(InputInterface $input, OutputInterface $output): int
    {
        dump($this->serializer->deserialize(<<<JSON
          {
            "list": {
              "test": 1,
              "2": 2
            }
          }
JSON, Foo::class, 'json'));

        return Command::SUCCESS;
    }
}

class Foo
{
    /**
     * @var list<string>
     */
    public array $list;
}