feat(interviews): LLM stub mode for deterministic CI tests

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/utils/llm_client.py

@@ -32,6 +32,31 @@ class LLMClient:
             base_url=self.base_url
         )
     
+    def _stub_key(self, messages: list[dict]) -> str:
+        user_msg = next((m["content"] for m in reversed(messages) if m.get("role") == "user"), "")
+        sys_msg = next((m["content"] for m in messages if m.get("role") == "system"), "")
+        # Allow callers to embed an explicit stub_key=... token
+        for chunk in user_msg.split():
+            if chunk.startswith("stub_key="):
+                return chunk[len("stub_key="):]
+        import hashlib
+        return hashlib.sha256((sys_msg + "|" + user_msg).encode("utf-8")).hexdigest()[:12]
+
+    def _stub_response(self, messages: list[dict]) -> str:
+        import json as _json
+        return _json.dumps(self._stub_response_json(messages), ensure_ascii=False)
+
+    def _stub_response_json(self, messages: list[dict]) -> dict:
+        key = self._stub_key(messages)
+        # Deterministic centered Likert + plausible open text
+        digit = sum(ord(c) for c in key) % 5 + 1
+        return {
+            "stub_key": key,
+            "responses": {"item_001": digit, "item_002": digit, "item_003": (digit % 5) + 1},
+            "confidence": {"item_001": 0.7, "item_002": 0.7, "item_003": 0.6},
+            "open_comment": f"stub:{key}",
+        }
+
     def chat(
         self,
         messages: List[Dict[str, str]],
@@ -41,16 +66,20 @@ class LLMClient:
     ) -> str:
         """
         发送聊天请求
-        
+
         Args:
             messages: 消息列表
             temperature: 温度参数
             max_tokens: 最大token数
             response_format: 响应格式（如JSON模式）
-            
+
         Returns:
             模型响应文本
         """
+        from app.config import Config
+        if getattr(Config, "LLM_STUB_MODE", False):
+            return self._stub_response(messages)
+
         kwargs = {
             "model": self.model,
             "messages": messages,
@@ -75,15 +104,19 @@ class LLMClient:
     ) -> Dict[str, Any]:
         """
         发送聊天请求并返回JSON
-        
+
         Args:
             messages: 消息列表
             temperature: 温度参数
             max_tokens: 最大token数
-            
+
         Returns:
             解析后的JSON对象
         """
+        from app.config import Config
+        if getattr(Config, "LLM_STUB_MODE", False):
+            return self._stub_response_json(messages)
+
         response = self.chat(
             messages=messages,
             temperature=temperature,

backend/tests/interviews/test_llm_stub.py

@@ -0,0 +1,17 @@
+import json
+from app.utils.llm_client import LLMClient
+
+
+def test_stub_mode_returns_deterministic_canned_json(monkeypatch):
+    monkeypatch.setenv("LLM_STUB_MODE", "true")
+    from app.config import Config
+    Config.LLM_STUB_MODE = True
+    client = LLMClient(api_key="x", base_url="x", model="x")
+    messages = [
+        {"role": "system", "content": "You are persona_42. Return JSON."},
+        {"role": "user", "content": "stub_key=longitudinal:item_001"},
+    ]
+    out1 = client.chat_json(messages=messages, temperature=0.0)
+    out2 = client.chat_json(messages=messages, temperature=0.0)
+    assert out1 == out2
+    assert isinstance(out1, dict)