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/unit/v1/agent/test_router_guards.py

@@ -225,3 +225,44 @@ async def test_stream_events_retries_on_redis_timeout(
 
     merged = "".join(chunks)
     assert "event: RUN_FINISHED" in merged
+
+
+@pytest.mark.asyncio
+async def test_get_attachment_preview_rejects_negative_index() -> None:
+    class _Service:
+        async def get_attachment_preview(self, **kwargs):  # noqa: ANN003
+            del kwargs
+            raise AssertionError("get_attachment_preview should not be called")
+
+    with pytest.raises(HTTPException) as exc_info:
+        await agent_router.get_attachment_preview(
+            thread_id="00000000-0000-0000-0000-000000000001",
+            message_id="00000000-0000-0000-0000-000000000010",
+            attachment_index=-1,
+            service=cast(Any, _Service()),
+            current_user=CurrentUser(id=uuid4(), email="user@example.com"),
+        )
+
+    assert exc_info.value.status_code == 422
+
+
+@pytest.mark.asyncio
+async def test_get_attachment_preview_returns_streaming_response() -> None:
+    class _Service:
+        async def get_attachment_preview(self, **kwargs):  # noqa: ANN003
+            del kwargs
+            return b"png-bytes", "image/png"
+
+    response = await agent_router.get_attachment_preview(
+        thread_id="00000000-0000-0000-0000-000000000001",
+        message_id="00000000-0000-0000-0000-000000000010",
+        attachment_index=0,
+        service=cast(Any, _Service()),
+        current_user=CurrentUser(id=uuid4(), email="user@example.com"),
+    )
+    chunks: list[bytes] = []
+    async for chunk in response.body_iterator:
+        chunks.append(cast(bytes, chunk))
+
+    assert response.media_type == "image/png"
+    assert b"".join(chunks) == b"png-bytes"