Return empty Record<string, unknown> for default context function

[sgohlke]

At the moment (version 4.x) we return the request object in the defaultContextFunction. This may cause issues like in https://github.com/dreamit-de/uexpress-example/actions/runs/12001332310/job/33451686378 where a readonly field query exists and setting it causes an unhandledRejection.

# Error: A resource generated asynchronous activity after the test ended. 
This activity created the error "TypeError: 
Cannot set property query of \#<Request> which has only a getter" which
triggered an unhandledRejection event, caught by the test runner.

I think the context should have the type Record<string, unknown> and the defaultContextFunction should return an empty Record so additional information can be properly added without manipulating the original request object.

This introduces a change in the default behaviour and thus a breaking change.