[Frontend] OpenAI Responses API supports Tool/Function calling - non-harmony (#26874)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

tests/v1/entrypoints/openai/serving_responses/test_structured_output.py

@@ -0,0 +1,78 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import json
+
+import openai
+import pytest
+from pydantic import BaseModel
+
+
+@pytest.mark.asyncio
+async def test_structured_output(client: openai.AsyncOpenAI):
+    response = await client.responses.create(
+        input=[
+            {"role": "system", "content": "Extract the event information."},
+            {
+                "role": "user",
+                "content": "Alice and Bob are going to a science fair on Friday.",
+            },
+        ],
+        text={
+            "format": {
+                "type": "json_schema",
+                "name": "calendar_event",
+                "schema": {
+                    "type": "object",
+                    "properties": {
+                        "event_name": {"type": "string"},
+                        "date": {"type": "string"},
+                        "participants": {"type": "array", "items": {"type": "string"}},
+                    },
+                    "required": ["event_name", "date", "participants"],
+                    "additionalProperties": False,
+                },
+                "description": "A calendar event.",
+                "strict": True,
+            }
+        },
+    )
+    print(response)
+
+    # NOTE: The JSON schema is applied to the output text, not reasoning.
+    output_text = response.output[-1].content[0].text
+    event = json.loads(output_text)
+
+    assert event["event_name"].lower() == "science fair"
+    assert event["date"] == "Friday"
+    participants = event["participants"]
+    assert len(participants) == 2
+    assert participants[0] == "Alice"
+    assert participants[1] == "Bob"
+
+
+@pytest.mark.asyncio
+async def test_structured_output_with_parse(client: openai.AsyncOpenAI):
+    class CalendarEvent(BaseModel):
+        event_name: str
+        date: str
+        participants: list[str]
+
+    response = await client.responses.parse(
+        model=None,
+        instructions="Extract the event information.",
+        input="Alice and Bob are going to a science fair on Friday.",
+        text_format=CalendarEvent,
+    )
+    print(response)
+
+    # The output is successfully parsed.
+    event = response.output_parsed
+    assert event is not None
+
+    # The output is correct.
+    assert event.event_name.lower() == "science fair"
+    assert event.date == "Friday"
+    participants = event.participants
+    assert len(participants) == 2
+    assert participants[0] == "Alice"
+    assert participants[1] == "Bob"