When user interrupts multipart download with Ctrl+C, display a helpful
message indicating checkpoint is saved and download can be resumed,
instead of returning an internal error with context.Canceled.
- Add multipart download engine (drive_transfer.go) with Range probe,
resume support, and credential auto-refresh on 401/403
- Add --part-size, --parallel, --no-resume flags to drive download and
download-version commands
- Replace httpGetFile with driveTransferDownload for chunked parallel
downloads in download and download-version commands
- Replace uploadToDrive credential parsing with driveUploadPut
(transparent header pass-through, retry on 401/403)
- Add typed httpStatusError for non-2xx HTTP responses in doc.go
- Add comprehensive unit tests (32 cases) for drive_transfer
- Update drive reference documentation with multipart download behavior
- Add E2E test for multipart download (auto-test/, gitignored)
CR: 28984991
Dennis