Start LangGraph backend migration foundation

test/agent/test_langgraph_backend.py

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+from copy import deepcopy
+
+import pytest
+
+from src.agent.state import AccidentGraphState, GeneratedTurn
+from src.backends.chat import ChatInput, FormUpdate, TextDelta
+from src.backends.fastgpt import FastGPTBackend
+from src.backends.langgraph import LangGraphBackend
+from src.core.config import Settings
+from src.core.fastgpt_client import create_chat_backend
+
+
+class FakeResponseGenerator:
+    def __init__(self):
+        self.states = []
+        self.closed = False
+
+    async def generate(self, state: AccidentGraphState) -> GeneratedTurn:
+        self.states.append(deepcopy(state))
+        turn_number = state.get("turn_count", 0) + 1
+        patch = {"turn": turn_number} if state["need_form_update"] else {}
+        return GeneratedTurn(
+            content=f"<state>1002</state>第{turn_number}轮",
+            form_update=patch,
+        )
+
+    async def aclose(self) -> None:
+        self.closed = True
+
+
+def langgraph_settings():
+    return Settings(
+        _env_file=None,
+        environment="test",
+        agent_backend="langgraph",
+        langgraph_checkpointer="memory",
+        llm_api_key="test-key",
+        llm_model="test-model",
+    )
+
+
+@pytest.mark.asyncio
+async def test_langgraph_backend_preserves_thread_scoped_turn_state():
+    generator = FakeResponseGenerator()
+    backend = create_chat_backend(
+        langgraph_settings(),
+        response_generator=generator,
+    )
+    assert isinstance(backend, LangGraphBackend)
+
+    first = await backend.complete(
+        ChatInput("session-1", "第一轮", need_form_update=True)
+    )
+    second = await backend.complete(
+        ChatInput("session-1", "第二轮", need_form_update=True)
+    )
+    other_session = await backend.complete(
+        ChatInput("session-2", "独立会话", need_form_update=True)
+    )
+
+    assert first.content == "<state>1002</state>第1轮"
+    assert first.form_update == {"turn": 1}
+    assert second.content == "<state>1002</state>第2轮"
+    assert second.form_update == {"turn": 2}
+    assert other_session.content == "<state>1002</state>第1轮"
+    assert generator.states[0].get("turn_count", 0) == 0
+    assert generator.states[1]["turn_count"] == 1
+    assert generator.states[2].get("turn_count", 0) == 0
+
+
+@pytest.mark.asyncio
+async def test_langgraph_stream_bridges_form_update_before_text():
+    generator = FakeResponseGenerator()
+    backend = create_chat_backend(
+        langgraph_settings(),
+        response_generator=generator,
+    )
+
+    events = [
+        event
+        async for event in backend.stream(
+            ChatInput("session-stream", "开始", need_form_update=True)
+        )
+    ]
+
+    assert events == [
+        FormUpdate({"turn": 1}),
+        TextDelta("<state>1002</state>第1轮"),
+    ]
+
+
+@pytest.mark.asyncio
+async def test_langgraph_backend_closes_owned_generator():
+    generator = FakeResponseGenerator()
+    backend = create_chat_backend(
+        langgraph_settings(),
+        response_generator=generator,
+    )
+
+    await backend.aclose()
+
+    assert generator.closed
+
+
+def test_factory_keeps_fastgpt_as_default_compatible_backend():
+    settings = Settings(
+        _env_file=None,
+        environment="test",
+        agent_backend="fastgpt",
+        fastgpt_api_key="test-key",
+        fastgpt_base_url="http://fastgpt.test",
+        fastgpt_app_id="test-app",
+    )
+    fake_client = object()
+
+    backend = create_chat_backend(
+        settings,
+        fastgpt_client=fake_client,
+    )
+
+    assert isinstance(backend, FastGPTBackend)