Fix duplicate get_supported_tasks definition in async_omni.py (#1825)

Signed-off-by: linyueqian <linyueqian@outlook.com>

tests/entrypoints/openai_api/test_serving_speech.py

@@ -1025,19 +1025,31 @@ class TestAsyncOmniSupportedTasks:
     @pytest.mark.asyncio
     async def test_tts_only_no_generate_task(self):
         """TTS-only models (audio output, no text) should not include 'generate'."""
+        from unittest.mock import MagicMock
+
         from vllm_omni.entrypoints.async_omni import AsyncOmni
 
         omni = AsyncOmni.__new__(AsyncOmni)
         omni.output_modalities = [None, "audio"]
+        stage = MagicMock()
+        stage.is_comprehension = False
+        omni.stage_list = [stage]
         tasks = await omni.get_supported_tasks()
         assert "generate" not in tasks
+        assert "speech" in tasks
 
     @pytest.mark.asyncio
     async def test_omni_model_includes_generate(self):
         """Models with text output (e.g. Qwen3-Omni) should include 'generate'."""
+        from unittest.mock import MagicMock
+
         from vllm_omni.entrypoints.async_omni import AsyncOmni
 
         omni = AsyncOmni.__new__(AsyncOmni)
         omni.output_modalities = ["text", None, "audio"]
+        stage = MagicMock()
+        stage.is_comprehension = True
+        omni.stage_list = [stage]
         tasks = await omni.get_supported_tasks()
         assert "generate" in tasks
+        assert "speech" in tasks