ReorderEligibilityConstraintMessageFormatter::format() can crash when returning non string value

[danakil]

ReorderEligibilityCheckers builds an array of parameters which is passed to $this->reorderEligibilityConstraintMessageFormatter->format(...)
For example, ReorderItemPricesEligibilityChecker build a $reorderProductNamesToTotal which uses product names as key.
If you have product names with only number ("123"), then the keys are casted to integer (123).

In ReorderEligibilityConstraintMessageFormatter::format(), we have:

`
if (count($messageParameters) === 1) {
$message = array_pop($messageParameters);

        return $message;
    }`

In this cas, its crashes because the return type is not a string but an integer.
Fix:
return (string) $message;