refactor: centralize llm backend configuration

plugins/game_task/config.toml

@@ -1,5 +1,6 @@
 [GameTask]
 enable = true
+backend = "openai_compatible_game_task"
 command = ["/t", "/a", "/s", "/r", "/l", "/h"]
 command-format = """
 🎮 百科问答指令：
@@ -10,8 +11,3 @@ command-format = """
 /l - 查看活跃任务
 /h - 查看未完成任务
 """
-
-# AI获取题目确认答案用的配置信息
-authorization = "Bearer b8586595-eb81-483d-8e91-a35cc789729e"  # 请替换为真实的Authorization token
-url = 'https://ark.cn-beijing.volces.com/api/v3/chat/completions'
-model = "doubao-1-5-lite-32k-250115"