fix: converge non-numeric selector scores to ValueError

tests/unit/test_distractor_selector.py

@@ -63,6 +63,14 @@ async def test_generate_pool_degrades_on_malformed_response():
     assert out == []
 
 
+@pytest.mark.asyncio
+async def test_score_options_raises_valueerror_on_non_numeric():
+    from app.question_gen.distractor_selector import _score_options
+    vlm = _FakeVLM(['{"scores": [0.9, null, 0.5]}'])  # null 不是数值
+    with pytest.raises(ValueError):
+        await _score_options(vlm, "?", ["蒸", "炒", "煮"], _Material(), session_id="s")
+
+
 @pytest.mark.asyncio
 async def test_build_grounded_options_happy_path():
     pool = '{"distractors": ["炒", "煮", "炸", "烤"]}'