[Feature][Responses API]Support MCP tools with streaming mode + background mode (#23927)

Signed-off-by: wuhang <wuhang6@huawei.com>

tests/entrypoints/openai/test_response_api_with_harmony.py

@@ -275,7 +275,8 @@ async def test_stateful_multi_turn(client: OpenAI, model_name: str):
 
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])
-async def test_streaming(client: OpenAI, model_name: str):
+@pytest.mark.parametrize("background", [True, False])
+async def test_streaming(client: OpenAI, model_name: str, background: bool):
     # TODO: Add back when web search and code interpreter are available in CI
     prompts = [
         "tell me a story about a cat in 20 words",
@@ -300,11 +301,16 @@ async def test_streaming(client: OpenAI, model_name: str):
                 # },
             ],
             stream=True,
+            background=background,
         )
 
         events = []
         current_event_mode = None
+        resp_id = None
         async for event in response:
+            if event.type == "response.created":
+                resp_id = event.response.id
+
             if current_event_mode != event.type:
                 current_event_mode = event.type
                 print(f"\n[{event.type}] ", end="", flush=True)
@@ -322,6 +328,17 @@ async def test_streaming(client: OpenAI, model_name: str):
 
         assert len(events) > 0
 
+        if background:
+            starting_after = 5
+            async with await client.responses.retrieve(
+                    response_id=resp_id,
+                    stream=True,
+                    starting_after=starting_after) as stream:
+                counter = starting_after
+                async for event in stream:
+                    counter += 1
+                    assert event == events[counter]
+
 
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])