HttpHelper returns *http.Response instead of []byte

- to provide the full response to consumers to e.g. parse status code
- consumers become responsible for error handling

[#85033736]
Remove unnecessary tests of empty-values in error cases.

[#85033736]
Client returns pointers rather than empty objects.

[#85033736]
Move ListTaskCount into its own file.

[#85033736]
Client checks for expected status codes.

[#85033736]