fix(release): retry snapshot bodies truncated mid-stream

http.client.IncompleteRead is not an OSError, so a response cut off
after streaming began escaped the retry handler on the first attempt
and left the partial file behind. Treat http.client transport
exceptions as retryable alongside the existing failures, and pin the
downloader's whole retry contract — truncation retried without residue,
5xx bursts bridged, client errors immediate, exhaustion fail-closed —
with a stubbed-opener harness run from the release test suite.

Co-authored-by: Cursor <cursoragent@cursor.com>