Review feedback: the previous test would pass even if the Range-header
logic in download_file were deleted entirely, because File::create
truncates the stale .part regardless of which branch set
actually_resuming to false.
Tightens spawn_no_range_server to return HTTP 400 when the request
carries no Range header, and only return 200 + full body when Range is
present. A regression that stops sending Range now surfaces as a
download failure (empty body from 400, bytes != body assertion)
instead of silently passing through the truncation path.
Jake
dd98cb7994