[Bugfix] fix streaming final output for non harmony (#30237)

Signed-off-by: penfree <qiupengfei@baidu.com>
Co-authored-by: penfree <qiupengfei@baidu.com>

tests/entrypoints/openai/test_response_api_simple.py

@@ -87,3 +87,48 @@ async def test_reasoning_item(client: OpenAI, model_name: str):
     assert response.output[0].type == "reasoning"
     assert response.output[1].type == "message"
     assert type(response.output[1].content[0].text) is str
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_streaming_output_consistency(client: OpenAI, model_name: str):
+    """Test that streaming delta text matches the final response output_text.
+
+    This test verifies that when using streaming mode:
+    1. The concatenated text from all 'response.output_text.delta' events
+    2. Matches the 'output_text' in the final 'response.completed' event
+    """
+    response = await client.responses.create(
+        model=model_name,
+        input="Say hello in one sentence.",
+        stream=True,
+    )
+
+    events = []
+    async for event in response:
+        events.append(event)
+
+    assert len(events) > 0
+
+    # Concatenate all delta text from streaming events
+    streaming_text = "".join(
+        event.delta for event in events if event.type == "response.output_text.delta"
+    )
+
+    # Get the final response from the last event
+    response_completed_event = events[-1]
+    assert response_completed_event.type == "response.completed"
+    assert response_completed_event.response.status == "completed"
+
+    # Get output_text from the final response
+    final_output_text = response_completed_event.response.output_text
+
+    # Verify final response has output
+    assert len(response_completed_event.response.output) > 0
+
+    # Verify streaming text matches final output_text
+    assert streaming_text == final_output_text, (
+        f"Streaming text does not match final output_text.\n"
+        f"Streaming: {streaming_text!r}\n"
+        f"Final: {final_output_text!r}"
+    )