feat: Agent 运行时、对话 API、作业助手与引擎修复及前端执行超时

- agent_runtime 模块与 agent_chat API，前端 AgentChat 视图与路由对接
- workflow_engine: code 节点命名空间与 json 引用修复
- llm_service: 工具调用 extra_body（如 DeepSeek）
- create_homework_manager_agent / _3 脚本与测试脚本扩展
- frontend: WORKFLOW_EXECUTION_HTTP_TIMEOUT_MS、AgentChatPreview/MainLayout 等
- 文档：架构说明与自主 Agent 改造完成情况

Made-with: Cursor

backend/app/agent_runtime/context.py

@@ -0,0 +1,87 @@
+"""
+Agent 会话上下文管理：维护消息历史、状态追踪。
+"""
+from __future__ import annotations
+
+import uuid
+from typing import Any, Dict, List, Optional
+
+
+class AgentContext:
+    """
+    Agent 会话上下文：
+
+    - 消息历史（messages 列表，OpenAI 格式）
+    - 会话元信息（session_id, user_id 等）
+    - 执行追踪（iteration 计数, 工具调用统计）
+    """
+
+    def __init__(
+        self,
+        system_prompt: str = "你是一个有用的AI助手。",
+        user_id: Optional[str] = None,
+        session_id: Optional[str] = None,
+    ):
+        self.session_id = session_id or str(uuid.uuid4())
+        self.user_id = user_id
+        self._messages: List[Dict[str, Any]] = []
+        self._system_prompt = system_prompt
+        # 执行状态
+        self.iteration = 0
+        self.tool_calls_made = 0
+
+    @property
+    def messages(self) -> List[Dict[str, Any]]:
+        """获取完整消息列表（含 system prompt）。"""
+        if self._system_prompt:
+            # 确保 system prompt 始终在第一条
+            has_system = (
+                len(self._messages) > 0
+                and self._messages[0].get("role") == "system"
+            )
+            if not has_system:
+                return [
+                    {"role": "system", "content": self._system_prompt},
+                    *self._messages,
+                ]
+        return self._messages
+
+    def add_user_message(self, content: str) -> None:
+        """添加用户消息。"""
+        self._messages.append({"role": "user", "content": content})
+
+    def add_assistant_message(
+        self,
+        content: str,
+        tool_calls: Optional[List[Dict[str, Any]]] = None,
+        reasoning_content: Optional[str] = None,
+    ) -> None:
+        """添加助手回复。"""
+        msg: Dict[str, Any] = {"role": "assistant", "content": content or ""}
+        if tool_calls:
+            msg["tool_calls"] = tool_calls
+        if reasoning_content:
+            msg["reasoning_content"] = reasoning_content
+        self._messages.append(msg)
+
+    def add_tool_result(
+        self, tool_call_id: str, tool_name: str, result: str
+    ) -> None:
+        """添加工具执行结果。"""
+        self._messages.append({
+            "role": "tool",
+            "tool_call_id": tool_call_id,
+            "content": result,
+            "name": tool_name,
+        })
+
+    def set_system_prompt(self, prompt: str) -> None:
+        """更新 system prompt（仅在未发送过消息时有效）。"""
+        if not self._messages:
+            self._system_prompt = prompt
+
+    def reset(self) -> None:
+        """重置上下文（保留 system prompt 和 session_id）。"""
+        self._messages = []
+        self.iteration = 0
+        self.tool_calls_made = 0