Bug: Model::getValidationRules() cant handle ::$validationRules if its a string

[element-code]

Describe the bug
When calling CodeIgniter\Model::getValidationRules() (as described here) we get a TypeError or a error from array_diff_key or array_intersect_key in case were using options.
This is caused by ::getValidationRules not checking for string / lookup like it happens in ::validate().

Always using ::getValidationRules instead of internal accessing them in ::validate() would be better and minimize the possibility of not checking for is_string :)

CodeIgniter 4 version
development

Affected module(s)
Codeigniter\Model