refactor(question_gen): extract _validate_parsed_fields to reduce CC

Split field validation logic out of _parse_v2_response into a dedicated
_validate_parsed_fields helper. This brings _parse_v2_response from CC=11
(grade C) down to CC=3 (grade A). The extracted validator is CC=9 (grade B).
No grade-C functions remain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/question_gen/generator_v2.py

@@ -220,6 +220,65 @@ def _extract_json_from_text(raw: str) -> str:
     return content
 
 
+@dataclass(frozen=True)
+class _ValidatedFields:
+    """字段校验通过后的中间结构。"""
+
+    question: str
+    options: tuple[str, ...]
+    answer: str
+    difficulty: str
+
+
+def _validate_parsed_fields(data: dict) -> _ValidatedFields:
+    """校验 VLM 响应 JSON 的必填字段并规范化。
+
+    参数:
+        data: 已解析的 JSON 字典。
+
+    返回:
+        _ValidatedFields 实例（字段已规范化）。
+
+    异常:
+        ValueError: 缺少必填字段或字段值非法。
+    """
+    missing = [f for f in ("question", "options", "answer", "difficulty") if f not in data]
+    if missing:
+        msg = f"VLM 响应缺少必填字段: {', '.join(missing)}"
+        raise ValueError(msg)
+
+    question_text = str(data["question"])
+    options_raw = data["options"]
+    answer = str(data["answer"]).strip().upper()
+    difficulty = str(data["difficulty"]).strip().lower()
+
+    # 校验 options
+    if not isinstance(options_raw, list) or len(options_raw) < 2:
+        msg = f"options 字段必须是至少 2 个选项的列表，实际: {options_raw}"
+        raise ValueError(msg)
+
+    # 校验 answer
+    if answer not in _VALID_ANSWERS:
+        msg = f"answer 字段值 '{answer}' 非法，必须为 A/B/C/D 之一"
+        raise ValueError(msg)
+
+    # 校验 difficulty（宽容处理：非法值回退为 medium）
+    if difficulty not in _VALID_DIFFICULTIES:
+        logger.warning(
+            "difficulty '{}' 不在预设范围 {}，回退为 'medium'",
+            difficulty,
+            _VALID_DIFFICULTIES,
+        )
+        difficulty = "medium"
+
+    return _ValidatedFields(
+        question=question_text,
+        options=tuple(str(o) for o in options_raw),
+        answer=answer,
+        difficulty=difficulty,
+    )
+
+
 def _parse_v2_response(
     raw: str,
     video_id: str,
@@ -233,7 +292,7 @@ def _parse_v2_response(
     流程：
     1. 提取 JSON（处理 markdown 包裹）。
     2. json_repair 修复常见格式错误。
-    3. 解析并校验必填字段。
+    3. 校验必填字段（委托 _validate_parsed_fields）。
     4. 构造 CandidateQuestion 实例。
 
     参数:
@@ -250,13 +309,11 @@ def _parse_v2_response(
     异常:
         ValueError: JSON 解析失败或缺少必填字段或字段值非法。
     """
-    # Phase 1: 提取 JSON 文本
+    # Phase 1: 提取 + 修复 JSON
     json_text = _extract_json_from_text(raw)
-
-    # Phase 2: json_repair 修复
     repaired = repair_json(json_text, return_objects=False)
 
-    # Phase 3: 解析
+    # Phase 2: 解析为字典
     try:
         data = json.loads(repaired)
     except json.JSONDecodeError as e:
@@ -267,38 +324,10 @@ def _parse_v2_response(
         msg = f"VLM 响应顶层不是 JSON 对象: type={type(data).__name__}"
         raise ValueError(msg)
 
-    # Phase 4: 校验必填字段
-    missing = [f for f in ("question", "options", "answer", "difficulty") if f not in data]
-    if missing:
-        msg = f"VLM 响应缺少必填字段: {', '.join(missing)}"
-        raise ValueError(msg)
+    # Phase 3: 字段校验
+    fields = _validate_parsed_fields(data)
 
-    question_text = str(data["question"])
-    options_raw = data["options"]
-    answer = str(data["answer"]).strip().upper()
-    difficulty = str(data["difficulty"]).strip().lower()
-
-    # 校验 options
-    if not isinstance(options_raw, list) or len(options_raw) < 2:
-        msg = f"options 字段必须是至少 2 个选项的列表，实际: {options_raw}"
-        raise ValueError(msg)
-    options = tuple(str(o) for o in options_raw)
-
-    # 校验 answer
-    if answer not in _VALID_ANSWERS:
-        msg = f"answer 字段值 '{answer}' 非法，必须为 A/B/C/D 之一"
-        raise ValueError(msg)
-
-    # 校验 difficulty
-    if difficulty not in _VALID_DIFFICULTIES:
-        logger.warning(
-            "difficulty '{}' 不在预设范围 {}，回退为 'medium'",
-            difficulty,
-            _VALID_DIFFICULTIES,
-        )
-        difficulty = "medium"
-
-    # Phase 5: 构造 CandidateQuestion
+    # Phase 4: 构造 CandidateQuestion
     question_id = f"{video_id}_{task_type}_{seq:04d}"
 
     return CandidateQuestion(
@@ -306,11 +335,11 @@ def _parse_v2_response(
         video_id=video_id,
         task_type=task_type,
         skill_target=skill_target,
-        question=question_text,
-        options=options,
-        answer=answer,
+        question=fields.question,
+        options=fields.options,
+        answer=fields.answer,
         source_nodes=source_nodes,
-        difficulty=difficulty,
+        difficulty=fields.difficulty,
     )