fix(request): keep response body readable

[pkurunner]

Summary

• stop closing http.Response.Body before returning ServerResponse
• add a regression test that reads the returned response body

Testing

• gofmt -w internal/request/request.go internal/request/response_body_test.go
• GOOS=linux GOARCH=amd64 go test -c ./internal/request
• Not run: go test ./internal/request on Windows is blocked by Unix-only rawhttp syscall usage.