refactor: Phase 1 — role-based strategy architecture with unified OSS (#353)

- Added AppType.OSSApp = 3 for OSS update mode
- Created OSSUpdateStrategy (implements IStrategy), absorbs old OSSStrategy logic
- Bootstrap now dispatches to OSS on AppType.OSSApp:
  client-side: download version config, compare, start upgrade process
  upgrade-side: read GlobalConfigInfoOSS from env, execute OSSUpdateStrategy
- Removed legacy files: OSSStrategy.cs, GeneralUpdateOSS.cs, GeneralClientOSS.cs
- OSS mode is now fully built-in — no separate entry classes needed

Closes #352

Author: JusterZhu

src/c#/GeneralUpdate.Core/Bootstrap/GeneralClientOSS.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;

src/c#/GeneralUpdate.Core/Bootstrap/GeneralUpdateBootstrap.cs

@@ -11,5 +11,11 @@ public class AppType
         /// upgrade program.
         /// </summary>
         public const int UpgradeApp = 2;
+
+        /// <summary>
+        /// OSS (Object Storage Service) update mode.
+        /// Downloads packages from cloud storage without a dedicated update server.
+        /// </summary>
+        public const int OSSApp = 3;
     }
-}
+}