fix(question_gen): 每次重试换视频避免同视频反复去重失败

Information Synopsis 163 道 benchmark 题，同一视频反复出题
必然相似。改为每次重试随机选不同视频。

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

tools/generate_questions.py

@@ -908,7 +908,12 @@ async def _run_generate(args: argparse.Namespace) -> None:
         exemplars = _select_exemplars(benchmark, task_type, 3, rng)
 
         for seq in range(start_seq, per_type):
-            # 随机选一个视频
+            used_node_ids: set[str] = set()
+            session_id = f"gen-{task_type}-{seq}"
+            generated = False
+
+            for _attempt in range(max_retries):
+                # 每次重试换一个视频，避免同视频反复去重失败
                 video_id = rng.choice(video_ids)
                 tree_path = videos_dir / video_id / "tree.json"
 
@@ -916,10 +921,8 @@ async def _run_generate(args: argparse.Namespace) -> None:
                     tree = TreeIndex.load_json(str(tree_path))
                 except Exception as exc:
                     logger.warning("加载树 {} 失败: {}", tree_path, exc)
-                total_failed += 1
                     continue
 
-            # frame_path 是相对于视频目录的，拼为绝对路径供 VLM 读取
                 video_dir = videos_dir / video_id
                 for l1 in tree.roots:
                     for l2 in l1.children:
@@ -927,11 +930,6 @@ async def _run_generate(args: argparse.Namespace) -> None:
                             if l3.frame_path and not Path(l3.frame_path).is_absolute():
                                 l3.frame_path = str(video_dir / l3.frame_path)
 
-            used_node_ids: set[str] = set()
-            session_id = f"gen-{task_type}-{seq}"
-            generated = False
-
-            for _attempt in range(max_retries):
                 candidate = await _generate_with_sem(
                     vlm,
                     tree,