test(update): add #293 e2e resume test for unknown-hint + monotonic progress

Adds an end-to-end test that drives the real `download_asset_with_resume`
against a flaky TCP server in the most realistic #293 shape: the caller passes
no size hint (None), the connection drops every partial chunk, and the size is
learned from the server (Content-Length on the 200, Content-Range on the 206
resumes, like a GitHub release asset). Asserts the payload is fully
reconstructed, the total is learned, and progress callbacks never regress
across reconnects.