chore(deps): update Downloader to 5.9.8 (#10222)

Downloader 5.9.6 -> 5.9.8

5.9.7 adds RemoteFileInfo.ContentType. 5.9.8 fixes stop/dispose races:
a cancelled multi-chunk download could report a NullReferenceException
as its error, and a Dispose racing the completion signal could swallow
DownloadFileCompleted. Nothing DownloaderHelper calls changed its API.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>