将所有agent工具的explanation字段改为可选

修复Pydantic验证错误：QueryTransferHistoryInput的explanation字段为必需，但用户未提供。
修改了74个工具文件，将explanation字段从必需改为可选，默认值为None。

app/agent/tools/impl/update_subscribe.py

@@ -16,10 +16,8 @@ from app.schemas.types import EventType
 class UpdateSubscribeInput(BaseModel):
     """更新订阅工具的输入参数模型"""
 
-    explanation: str = Field(
-        ...,
-        description="Clear explanation of why this tool is being used in the current context",
-    )
+    explanation: Optional[str] = Field(None,
+        description="Clear explanation of why this tool is being used in the current context",)
     subscribe_id: int = Field(
         ...,
         description="The ID of the subscription to update (can be obtained from query_subscribes tool)",