[Core] Remove internal restriction for BotContext in BotSignProvider

Author: Linwenxuan04

Lagrange.Core/Common/BotSignProvider.cs

@@ -10,7 +10,7 @@ public abstract class BotSignProvider
 {
     private BotContext? _context;
 
-    protected internal BotContext Context
+    protected BotContext Context
     {
         get => _context ?? throw new InvalidOperationException("Context is not initialized.");
         set => _context = value;