Add FileInputBot for non-interactive debugging and fix NbtToString crash

FileInputBot (ChatBots/FileInputBot.cs):
- New ChatBot that monitors a text file (default: mcc_input.txt) for
  commands, enabling MCC control from Cursor Shell or any non-interactive
  environment where stdin is not available
- Activated by setting MCC_FILE_INPUT env var (e.g. MCC_FILE_INPUT=1)
- Polls every ~500ms for new lines appended to the file
- Lines starting with "/" are sent as server chat/commands
- Other lines are executed as MCC internal commands (same as console input)
- File path overridable via MCC_INPUT_FILE env var

McClient.cs:
- Load FileInputBot when MCC_FILE_INPUT environment variable is set

ChatParser.cs - NbtToString:
- Fix InvalidCastException when NBT "text" or nameless root tag values
  are Int32 instead of String (happens with 1.20.6 SystemChat packets
  containing numeric values in the chat component tree)
- Replace direct (string) casts with ?.ToString() ?? string.Empty

Made-with: Cursor

MinecraftClient/Protocol/Message/ChatParser.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -510,8 +510,7 @@ namespace MinecraftClient.Protocol.Message
         {
             if (nbt.Count == 1 && nbt.TryGetValue("", out object? rootMessage))
             {
-                // Nameless root tag
-                return (string)rootMessage;
+                return rootMessage?.ToString() ?? string.Empty;
             }
 
             string message = string.Empty;
@@ -526,7 +525,7 @@ namespace MinecraftClient.Protocol.Message
                 {
                     case "text":
                     {
-                        message = (string)value;
+                        message = value?.ToString() ?? string.Empty;
                     }
                         break;
                     case "extra":