Just asking: does it really make sense for the Serializer and is it consistent to blindly cast the values?
AFAIK, the object normalizer:

• returns the value if it matches is_* functions:

  symfony/src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

  Lines 436 to 438 in b1bee60

  	if (('is_'.$builtinType)($data)) {
  	return $data;
  	}

• throws on non-matching types:

  symfony/src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

  Line 445 in b1bee60

  throw new NotNormalizableValueException(sprintf('The type of the "%s" attribute for class "%s" must be one of "%s" ("%s" given).', $attribute, $currentClass, implode('", "', array_keys($expectedTypes)), \gettype($data)));

• unless the DISABLE_TYPE_ENFORCEMENT flag is set:

  symfony/src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

  Lines 441 to 443 in b1bee60

  	if ($context[self::DISABLE_TYPE_ENFORCEMENT] ?? $this->defaultContext[self::DISABLE_TYPE_ENFORCEMENT] ?? false) {
  	return $data;
  	}

  then, it returns the original data and let the validation occur after deserialization.

@ogizanagi object normalizer doesn't work with scalar values. Data that passed into denormalize method should be an object.

Thx for your patience @a-menshchikov