Improve shared examples for api admin mode with failed status code

What does this MR do and why?

Further improvements for the shared examples for the api admin mode concerning failed_status_code. A test was adapted accordingly to test the function.

Basicly: It should be possible to change the failed status code, e.g. instead to check for status :forbidden it should check for e.g. :not_found

How to:

it_behaves_like 'GET request permissions for admin mode', :not_found

This results in (for a regular user)

admin mode	http method	outcome (status)
true	GET	:not_found
false	GET	:not_found

This results in (for an admin)

admin mode	http method	outcome (status)
true	GET	:ok
false	GET	:not_found

Context: issue

How should the shared examples behave: #42692 (comment 1299073958)

🛠 with ❤ at Siemens

How to set up and validate locally

Either adjust any test from spec/spec_helper.rb using the shared examples OR run spec/requests/api/freeze_periods_spec.rb

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