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

tests/unit/test_sampler_v2.py

@@ -291,6 +291,21 @@ class TestSampleMaterialV2:
         has_related = any(pattern in s for s in result.subtitle_sentences)
         assert has_related
 
+    def test_invalid_level_raises(self, real_tree: TreeIndex) -> None:
+        """无效的 level 参数应抛出 ValueError。"""
+        from app.question_gen.sampler_v2 import sample_material_v2
+
+        rng = random.Random(42)
+        with pytest.raises(ValueError, match="level 必须为"):
+            sample_material_v2(
+                tree=real_tree,
+                task_type="Object Recognition",
+                used_node_ids=set(),
+                rng=rng,
+                level=4,
+                constraint=RETRIEVAL_FAMILY.sampling,
+            )
+
 
 class TestValidateSamplingConstraints:
     """_validate_sampling_constraints 辅助函数测试。"""