refactor: 重构 schemas 结构，统一枚举定义

backend/tests/unit/v1/agent/test_service.py

@@ -12,7 +12,7 @@ import pytest
 import v1.agent.service as agent_service_module
 from core.auth.models import CurrentUser
 from core.config.settings import config
-from schemas.messages.chat_message import AgentChatMessageMetadata
+from schemas.domain.chat_message import AgentChatMessageMetadata
 from v1.agent.service import AgentService