This will be useful at some point.

DoctrineArgumentResolver.php

<?php declare(strict_types=1);

namespace App\Request\ArgumentResolver;

use Doctrine\Persistence\ManagerRegistry;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Controller\ArgumentValueResolverInterface;
use Symfony\Component\HttpKernel\ControllerMetadata\ArgumentMetadata;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class DoctrineArgumentResolver implements ArgumentValueResolverInterface
{
    public function __construct(
        private readonly ?ManagerRegistry $registry = null,
    ) {
    }

    public function supports(Request $request, ArgumentMetadata $argument): bool
    {
        // If there is no manager, only Doctrine DBAL is configured. And we're only working with Doctrine entities,
        // which are objects, which we expect to be managed by an entity manager.
        if ($argument->isVariadic()
            || null === $this->registry
            || !class_exists($argument->getType(), false)
            || null === $entityManager = $this->registry->getManagerForClass($argument->getType())
        ) {
            return false;
        }
        if (!$argument->isNullable()
            && (!$argument->hasDefaultValue() || $argument->getDefaultValue() === null)
            && $request->attributes->get($argument->getName()) === null
        ) {
            return false;
        }
        return !$entityManager->getMetadataFactory()->isTransient($argument->getType());
    }

    public function resolve(Request $request, ArgumentMetadata $argument): iterable
    {
        try {
            $argumentValue = $request->attributes->get($argument->getName()) ?? $argument->getDefaultValue();
        } catch (\LogicException) {
            $argumentValue = null;
        }
        $entityRepository = $this->registry
            ->getManagerForClass($argument->getType())
            ->getRepository($argument->getType());
        $entity = $entityRepository->find($argumentValue)
            ?? $entityRepository->findOneBy([$argument->getName() => $argumentValue]);
        if ($entity === null && !$argument->isNullable()) {
            throw new NotFoundHttpException(sprintf('Doctrine entity "%s" not found.', $argument->getType()));
        }
        yield $entity;
    }
}