refactor: Fix phpstan codeigniter.modelArgumentInstanceOf

[neznaika0]

Description
I need help.
I still don't understand how to work with generics. I can add simple hints, but any condition raises questions.

How do I specify here "If $which is class-string<Model> ? class-string<Model> : Model|object|string|null" and then update the hint for model($this->modelName)

The phpstan documentation is small. There is no description of most use cases. Did I miss something?

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide