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).

src/core/fastgpt_client.py

@@ -2,8 +2,10 @@
 from contextlib import asynccontextmanager
 from fastapi import FastAPI
 from fastgpt_client import AsyncChatClient
+from ..agent.graph import build_graph
 from ..backends.chat import ChatBackend
 from ..backends.fastgpt import FastGPTBackend
+from ..backends.langgraph import LangGraphBackend
 from .config import Config
 
 
@@ -13,20 +15,25 @@ _chat_backend: ChatBackend | None = None
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    """Manage FastGPT client lifecycle."""
+    """Manage chat backend resource lifecycle."""
     global _chat_backend, _fastgpt_client
     Config.validate()
 
-    _fastgpt_client = AsyncChatClient(
-        api_key=Config.FASTGPT_API_KEY,
-        base_url=Config.FASTGPT_BASE_URL,
-        timeout=60.0,
-        max_retries=3,
-        retry_delay=1.0,
-        enable_logging=Config.DEBUG,
-    )
-    await _fastgpt_client.__aenter__()
-    _chat_backend = FastGPTBackend(_fastgpt_client)
+    if Config.AGENT_BACKEND == "langgraph":
+        # 作业 2：LangGraph fake 模式不要求 FastGPT 密钥。
+        # TODO(作业 2): 若语音仍启用，需在此单独初始化并校验 FastGPT 配置。
+        _chat_backend = LangGraphBackend(build_graph())
+    else:
+        _fastgpt_client = AsyncChatClient(
+            api_key=Config.FASTGPT_API_KEY,
+            base_url=Config.FASTGPT_BASE_URL,
+            timeout=60.0,
+            max_retries=3,
+            retry_delay=1.0,
+            enable_logging=Config.DEBUG,
+        )
+        await _fastgpt_client.__aenter__()
+        _chat_backend = FastGPTBackend(_fastgpt_client)
 
     try:
         yield