[bugfix] Qwen3-VL fix video incorrect timestamp calculations while do_sample_frames=True (#27104)

Co-authored-by: 松灵 <wpf272043@alibaba-inc.com>

vllm/model_executor/models/qwen3_vl.py

@@ -735,9 +735,9 @@ class Qwen3VLProcessingInfo(Qwen2VLProcessingInfo):
         if do_sample_frames:
             # here video_fps is the fps of the sampled video, and
             # metadata["fps"] refers to the fps of the original video.
-            video_fps = sampled_fps if sampled_fps else video_processor.fps
+            sampled_fps = sampled_fps if sampled_fps else video_processor.fps
             total_num_frames = metadata["total_num_frames"]
-            num_frames = int(total_num_frames / metadata["fps"] * video_fps)
+            num_frames = int(total_num_frames / metadata["fps"] * sampled_fps)
             num_frames = min(
                 min(
                     max(num_frames, video_processor.min_frames),