feat: normalize internal system user ID in notification dispatch

- Add SYSTEM_INTERNAL_USER_ID constant and helpers to app.utils.identity
- Ensure internal user ID is normalized to None before dispatching notifications, preventing misrouting to external channels
- Refactor MessageChain to use normalization for all message dispatch methods
- Add tests for internal user ID normalization and notification dispatch behavior

app/utils/identity.py

@@ -0,0 +1,27 @@
+from typing import Optional, Union
+
+# 后台任务会话使用的内部占位用户ID。
+# 它只用于在 agent/memory/session 侧标识“系统触发的任务”，
+# 不能直接作为真实消息接收人下发到 Telegram/企业微信 等通知渠道。
+SYSTEM_INTERNAL_USER_ID = "system"
+
+
+def is_internal_user_id(userid: Optional[Union[str, int]]) -> bool:
+    """
+    判断是否为系统内部占位用户ID。
+    """
+    return (
+        isinstance(userid, str)
+        and userid.strip().lower() == SYSTEM_INTERNAL_USER_ID
+    )
+
+
+def normalize_internal_user_id(
+    userid: Optional[Union[str, int]]
+) -> Optional[Union[str, int]]:
+    """
+    将系统内部占位用户ID归一化为 None，避免被通知渠道误认为真实接收人。
+    """
+    if is_internal_user_id(userid):
+        return None
+    return userid