feat(backends): wire LangGraph backend behind AGENT_BACKEND switch (assignment 2)

- src/backends/langgraph.py: LangGraphBackend implements ChatBackend;
  complete() runs one graph turn per session (thread_id=sessionId) and
  returns state-prefixed content; stream() emits the full state prefix
  before body fragments (full-answer slicing, not yet real incremental
  output - assignment 8)
- src/core/config.py: AGENT_BACKEND=fastgpt|langgraph, langgraph mode
  no longer requires FastGPT credentials at startup
- src/core/fastgpt_client.py: lifespan builds the backend by switch
- test/backends/test_langgraph_backend.py: adapter acceptance tests

Verified:
- pytest: 48 passed
- AGENT_BACKEND=langgraph uvicorn: POST /chat -> 1002;
  browser_addon -> browser_addon.1002; SSE = stage_code, text_delta,
  single done

Known limit: stream slices a complete answer; no formUpdate deltas yet
(assignment 6).

test/backends/test_langgraph_backend.py

@@ -0,0 +1,62 @@
+"""作业 2 验收测试：LangGraph 后端适配器。
+
+通过标准（对应迁移计划作业 2）：
+- ``complete()`` 返回带 ``<state>1002</state>`` 前缀的 ``ChatResult``，
+  API 层据此解析阶段码；
+- ``stream()`` 先产出含完整 state 前缀的 ``TextDelta``，再产出正文，
+  保证 ``stage_code`` 事件先于正文；
+- 会话通过 ``thread_id = session_id`` 绑定（图 checkpoint 语义）。
+
+注意：``src/backends/langgraph.py`` 中的 TODO 完成前，
+本文件的测试会失败，属于预期状态。
+"""
+
+import pytest
+
+from src.backends.chat import ChatInput, TextDelta
+from src.backends.langgraph import LangGraphBackend
+
+
+def make_input() -> ChatInput:
+    return ChatInput(
+        session_id="session-001",
+        text="发生了交通事故",
+        client_mode="direct",
+    )
+
+
+@pytest.mark.asyncio
+async def test_complete_returns_state_prefixed_result() -> None:
+    backend = LangGraphBackend()
+
+    result = await backend.complete(make_input())
+
+    assert result.content.startswith("<state>1002</state>")
+    assert "发生了交通事故" in result.content
+
+
+@pytest.mark.asyncio
+async def test_stream_emits_prefix_before_body() -> None:
+    backend = LangGraphBackend()
+
+    events = [event async for event in backend.stream(make_input())]
+
+    assert events, "stream 至少应产生一个 TextDelta"
+    assert all(isinstance(event, TextDelta) for event in events)
+    joined = "".join(event.text for event in events)
+    assert joined.startswith("<state>1002</state>")
+    assert "发生了交通事故" in joined
+
+
+@pytest.mark.asyncio
+async def test_sessions_are_isolated_by_thread_id() -> None:
+    backend = LangGraphBackend()
+
+    first = await backend.complete(make_input())
+    second = await backend.complete(make_input())
+    other = await backend.complete(
+        ChatInput(session_id="session-002", text="你好", client_mode="direct")
+    )
+
+    assert first.content != "" and second.content != ""
+    assert "你好" in other.content