test(transport): satisfy bodyclose in etag_test helpers

The do/helper closures returned *http.Response, which the bodyclose
linter flags at each call site even though the body is closed inside
the closure. Return only the asserted values (status code, body, and
headers) so no response escapes the helper.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4959e1f9-f8e6-4e97-a487-f395a0123c79