chore: Clean up redundant code and align model signatures

[JusterZhu]

Summary

Clean up leftover artifacts and align model signatures with the refactoring plan.

Tasks

1. Remove empty GeneralUpdate.ClientCore directory

• The csproj was already removed; only bin/ and obj/ build artifacts remain
• Path: src/c#/GeneralUpdate.ClientCore/

2. Align DownloadResult with plan

• Current: DownloadResult(string? Url, string? LocalPath, long DownloadedBytes, TimeSpan Duration, int RetryCount, bool Success, string? ErrorMessage)
• Expected: DownloadResult(DownloadAsset Asset, string LocalPath, long DownloadedBytes, TimeSpan Duration, int RetryCount, bool Success, string? ErrorMessage)
• Benefit: callers get full DownloadAsset info (Name, Version, SHA256) in download callbacks

3. Align IDownloadExecutor.ExecuteAsync signature

• Current: ExecuteAsync(string url, string destPath, IProgress<DownloadProgress>? progress, CancellationToken token)
• Expected: ExecuteAsync(DownloadAsset asset, string destPath, IProgress<DownloadProgress>? progress, CancellationToken token)
• Pass the full DownloadAsset instead of just the URL

4. Clean stale build artifacts

• Remove bin/Debug/net9.0/ directories (not in csproj target frameworks)

Ref

Refactoring plan v2: Section 8.4, Sub Issue 4