see symfony/symfony#27917 + symfony/symfony#12312

https://symfony.com/doc/current/validation/custom_constraint.html

instead of

if (!is_string($value)) {
-    throw new UnexpectedTypeException($value, 'string');
+    throw new UnexpectedValueException($value, 'string');
}

This will cause a violation to be raised instead.

The "string" value is used with This value should be of type {{ type }}., thus should be translator friendly. E.g. core uses phpdoc notation to avoid any grammar being used here (string|int instead of string or integer and Some&Other instead of Some and Other).