Preserve update logs in precheck update info

[Copilot]

AddListenerUpdatePrecheck received update version metadata without UpdateLog, even when the server response included updateLog. The field was lost while converting VersionEntry into DownloadAsset, then reconstructing VersionEntry for event dispatch.

• Carry release notes through the download model

  • Added DownloadAsset.UpdateLog as an init-only property.
  • Preserves existing DownloadAsset constructor compatibility.

• Preserve mappings across the update pipeline

  • HttpDownloadSource.MapVersionEntry now copies VersionEntry.UpdateLog.
  • ClientStrategy now copies DownloadAsset.UpdateLog when building UpdateInfoEventArgs.

• Regression coverage

  • Covers VersionEntry → DownloadAsset mapping.
  • Covers ClientStrategy.ExecuteAsync precheck callback receiving UpdateLog.

var versionInfos = downloadPlan.Assets.Select(a => new VersionEntry
{
    Version = a.Version,
    UpdateLog = a.UpdateLog,
    // existing fields...
}).ToList();

• Fixes [ERROR] AddListenerUpdatePrecheck中updateInfo拿不到UpdateLog，UpdateLog为空 #534