[Bugfix][Model] Fix Qwen3-Omni video metadata handling for use_audio_in_video (#4959)

Signed-off-by: ZhengWG <zwg0606@gmail.com>

tests/e2e/online_serving/test_qwen3_omni_expansion.py

@@ -410,6 +410,44 @@ def test_audio_in_video_002(omni_server, openai_client) -> None:
     openai_client.send_omni_request(request_config, request_num=get_max_batch_size())
 
 
+@hardware_test(res={"cuda": "H100", "rocm": "MI325"}, num_cards=2)
+@pytest.mark.parametrize("omni_server", test_params, indirect=True)
+def test_audio_in_video_default_loader_sampling_regression(omni_server, openai_client) -> None:
+    """
+    Regression: ``use_audio_in_video`` with default video loader sampling (no
+    ``media_io_kwargs.video.fps``) after a prior request that pinned fps.
+
+    Before the fix the second request could partial-hit mm-cache (audio cached,
+    video re-processed without audio) and fail with ``StopIteration`` in
+    ``replace_multimodal_special_tokens``.
+    """
+    video_data_url = f"data:video/mp4;base64,{generate_synthetic_video(224, 224, 128, embed_audio=True)['base64']}"
+    messages = dummy_messages_from_mix_data(
+        system_prompt=get_system_prompt(),
+        video_data_url=video_data_url,
+        content_text="Reply with one word: OK.",
+    )
+
+    base_config = {
+        "model": omni_server.model,
+        "messages": messages,
+        "stream": False,
+        "use_audio_in_video": True,
+        "modalities": ["text"],
+    }
+
+    # Prime mm-cache with explicit loader fps (historical failure mode).
+    openai_client.send_omni_request(
+        {
+            **base_config,
+            "extra_body": {"media_io_kwargs": {"video": {"fps": 1, "num_frames": 128}}},
+        }
+    )
+
+    # Default loader sampling: must succeed without explicit media_io fps.
+    openai_client.send_omni_request(dict(base_config))
+
+
 @hardware_test(res={"cuda": "H100", "rocm": "MI325"}, num_cards=2)
 @pytest.mark.parametrize("omni_server", test_params, indirect=True)
 def test_one_word_prompt_001(omni_server, openai_client) -> None: