Batch 10: OSS switch to new download + remove old SilentUpdateMode and DownloadManager

[JusterZhu]

Batch 10: Final Cleanup

Tasks

1. OSSUpdateStrategy → new download abstractions — Replace old DownloadManager calls with HttpDownloadSource + DownloadPlanBuilder + DefaultDownloadOrchestrator

2. Delete SilentUpdateMode — Already marked [Obsolete], replaced by SilentPollOrchestrator

3. Delete DownloadManager + DownloadTask — Remove after confirming no remaining references

Related

• Refactor plan: GeneralUpdate-refactor-plan.md