Round 3: Unit tests for Download orchestrator, HttpDownloadExecutor, VersionService, SilentPollOrchestrator, and Download sources

[JusterZhu]

Overview

Follow-up to #423 → #424 → #425 → #426. Final round covering the remaining core modules.

Modules to cover

Module	Priority
DefaultDownloadOrchestrator (ExecuteAsync full flow)	High
HttpDownloadExecutor (Range/Resume/progress)	High
DownloadProgressReporter	Medium
VersionService (retry/timeout/auth)	High
SilentPollOrchestrator	Medium
HttpDownloadSource / HubDownloadSource / OssDownloadSource	Medium

Test Pattern

• AAA (Arrange-Act-Assert)
• xUnit + Moq
• All tests passing on net10.0