fix(sampler): validate level param rejects invalid values

Add ValueError guard at the top of sample_material_v2 for level not in
{1, 2, 3}, preventing silent fallthrough to L1 sampling. Add unit test
for the new validation.

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

app/question_gen/sampler_v2.py

@@ -525,8 +525,12 @@ def sample_material_v2(
         MaterialContext 实例。
 
     异常:
+        ValueError: level 不在 {1, 2, 3} 中。
         RuntimeError: 耗尽 max_attempts 次尝试仍无法满足约束。
     """
+    if level not in (1, 2, 3):
+        msg = f"level 必须为 1、2 或 3，收到: {level}"
+        raise ValueError(msg)
 
     for attempt in range(max_attempts):
         # Phase 1: 按层级采样候选节点