[gpt-oss] Add ResponseReasoningPartAddedEvent, ResponseReasoningPartDoneEvent for streaming (#24938)

Signed-off-by: Andrew Xia <axia@meta.com>

tests/entrypoints/openai/test_response_api_with_harmony.py

@@ -287,6 +287,57 @@ async def test_stateful_multi_turn(client: OpenAI, model_name: str):
     assert response3.status == "completed"
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("model_name", [MODEL_NAME])
+async def test_streaming_types(client: OpenAI, model_name: str):
+    prompts = [
+        "tell me a story about a cat in 20 words",
+    ]
+
+    # this links the "done" type with the "start" type
+    # so every "done" type should have a corresponding "start" type
+    # and every open block should be closed by the end of the stream
+    pairs_of_event_types = {
+        "response.completed": "response.created",
+        "response.output_item.done": "response.output_item.added",
+        "response.content_part.done": "response.content_part.added",
+        "response.output_text.done": "response.output_text.delta",
+        "response.web_search_call.done": "response.web_search_call.added",
+        "response.reasoning_text.done": "response.reasoning_text.delta",
+        "response.reasoning_part.done": "response.reasoning_part.added",
+    }
+
+    for prompt in prompts:
+        response = await client.responses.create(
+            model=model_name,
+            input=prompt,
+            reasoning={"effort": "low"},
+            tools=[],
+            stream=True,
+            background=False,
+        )
+
+        stack_of_event_types = []
+        async for event in response:
+            if event.type == 'response.created':
+                stack_of_event_types.append(event.type)
+            elif event.type == 'response.completed':
+                assert stack_of_event_types[-1] == pairs_of_event_types[
+                    event.type]
+                stack_of_event_types.pop()
+            if event.type.endswith("added"):
+                stack_of_event_types.append(event.type)
+            elif event.type.endswith("delta"):
+                if stack_of_event_types[-1] == event.type:
+                    continue
+                stack_of_event_types.append(event.type)
+            elif event.type.endswith("done"):
+                assert stack_of_event_types[-1] == pairs_of_event_types[
+                    event.type]
+                stack_of_event_types.pop()
+        assert len(stack_of_event_types) == 0
+
+
 @pytest.mark.asyncio
 @pytest.mark.parametrize("model_name", [MODEL_NAME])
 @pytest.mark.parametrize("background", [True, False])
@@ -343,7 +394,10 @@ async def test_streaming(client: OpenAI, model_name: str, background: bool):
                 assert event.item_id == current_item_id
 
             # verify content_index_id is correct
-            if event.type == "response.content_part.added":
+            if event.type in [
+                    "response.content_part.added",
+                    "response.reasoning_part.added"
+            ]:
                 assert event.content_index != current_content_index
                 current_content_index = event.content_index
             elif event.type in [