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/config.py

@@ -15,9 +15,21 @@ class Config:
     FASTGPT_BASE_URL = os.getenv("ANALYSIS_SERVICE_URL")
     FASTGPT_APP_ID = os.getenv("APP_ID")
     
+    # Chat backend switch（作业 2）
+    # "fastgpt" 保持现有行为；"langgraph" 使用内存图 fake 回复。
+    AGENT_BACKEND = os.getenv("AGENT_BACKEND", "fastgpt")
+    
     @classmethod
     def validate(cls):
         """Validate required configuration."""
+        # TODO(作业 2): 语音管线仍依赖 FastGPT。当前仅按文本后端开关校验；
+        # 接入语音开关后，需要单独校验语音路径所需配置。
+        if cls.AGENT_BACKEND not in ("fastgpt", "langgraph"):
+            raise ValueError(
+                f"AGENT_BACKEND must be 'fastgpt' or 'langgraph', got {cls.AGENT_BACKEND!r}"
+            )
+        if cls.AGENT_BACKEND != "fastgpt":
+            return
         required = [
             ("FASTGPT_BASE_URL (ANALYSIS_SERVICE_URL)", cls.FASTGPT_BASE_URL),
             ("FASTGPT_API_KEY (ANALYSIS_AUTH_TOKEN)", cls.FASTGPT_API_KEY),