Request::str() and Request::string() return types

[shaffe-fr]

Hi,
The PR #42118 introduced the Request::str and Request::string methods.
I think it's a good addition for strict typing.
It make sense that like for the str helper the Request::str() returns a Stringable.
However, I find not very intuitive that string doesn't return an actual string but a Stringable. We'll often have to cast as (string), kinda loosing the goal of adding the method.

What do you think about that? Should we change Request::string return type to string?
Should I create a [10.x] PR?

Thanks