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

@@ -908,30 +908,28 @@ 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
-            video_id = rng.choice(video_ids)
-            tree_path = videos_dir / video_id / "tree.json"
-
-            try:
-                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:
-                    for l3 in l2.children:
-                        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):
+                # 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"
+
+                try:
+                    tree = TreeIndex.load_json(str(tree_path))
+                except Exception as exc:
+                    logger.warning("加载树 {} 失败: {}", tree_path, exc)
+                    continue
+
+                video_dir = videos_dir / video_id
+                for l1 in tree.roots:
+                    for l2 in l1.children:
+                        for l3 in l2.children:
+                            if l3.frame_path and not Path(l3.frame_path).is_absolute():
+                                l3.frame_path = str(video_dir / l3.frame_path)
+
                 candidate = await _generate_with_sem(
                     vlm,
                     tree,