feat: 添加 Agent 步骤事件与图片附件功能

- 新增 stepStarted/stepFinished 事件类型支持
- 前端实现图片附件上传和预览功能
- 后端增强工具结果存储和事件处理
- 完善相关单元测试和集成测试

backend/tests/integration/test_auth_routes.py

@@ -1,16 +1,13 @@
 from __future__ import annotations
 
 from typing import Callable
-from uuid import UUID
 
 import pytest
 from fastapi import HTTPException
 from fastapi.testclient import TestClient
 
 from app import app
-from core.auth.models import CurrentUser
 from v1.auth.dependencies import get_auth_service
-from v1.users.dependencies import get_current_user
 from v1.auth.rate_limit import reset_rate_limit_state
 from v1.auth.schemas import (
     AuthUser,