fix(DiffPipeline): CopyUnknownFiles 使用原子替换避免文件占用冲突

ISSUE: 全量包升级时，CopyUnknownFiles 直接调用 File.Copy(dest, true)
覆盖目标文件。若目标进程刚退出，文件句柄未释放，抛出 IOException:
'The process cannot access the file because it is being used by another process'

导致 AllPackagesSucceeded=false，版本号不回写 manifest，ClientTest
陷入无限循环检测-下载-失败的更新循环。

FIX: CopyUnknownFiles 改为和 ApplyPatch 一致的 temp-file 原子替换策略：
  1. File.Copy 到临时文件 {random}_{filename}
  2. File.Replace 原子替换目标文件
  3. 新文件直接用 File.Move

File.Replace 是 NTFS 内核级原子操作，不会出现半覆盖状态。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu

src/c#/GeneralUpdate.Core/Pipeline/DiffPipeline.cs

@@ -507,7 +507,22 @@ private static Task CopyUnknownFiles(string appPath, string patchPath)
                 if (parentFolder?.Exists == false)
                     parentFolder.Create();
 
-                File.Copy(file.FullName, targetPath, true);
+                // Atomic replace via temp file, same strategy as ApplyPatch.
+                // Avoids file-in-use errors when the process just exited.
+                var safeName = targetFileName.Replace(Path.DirectorySeparatorChar, '_');
+                var tempPath = Path.Combine(appPath, $"{Path.GetRandomFileName()}_{safeName}");
+                File.Copy(file.FullName, tempPath, true);
+
+                // Use File.Replace for atomic overwrite (handles locked targets gracefully)
+                if (File.Exists(targetPath))
+                {
+                    File.SetAttributes(targetPath, FileAttributes.Normal);
+                    File.Replace(tempPath, targetPath, null, true);
+                }
+                else
+                {
+                    File.Move(tempPath, targetPath);
+                }
             }
 
             if (Directory.Exists(patchPath))