Implement OpenAI Responses API [1/N] (#20504)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/v1/entrypoints/openai/responses/test_basic.py

@@ -0,0 +1,75 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import openai  # use the official client for correctness check
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_simple_input(client: openai.AsyncOpenAI):
+    response = await client.responses.create(input="What is 13 * 24?")
+    print(response)
+
+    outputs = response.output
+    # Whether the output contains the answer.
+    assert outputs[-1].type == "message"
+    assert "312" in outputs[-1].content[0].text
+
+    # Whether the output contains the reasoning.
+    assert outputs[0].type == "reasoning"
+    assert outputs[0].text != ""
+
+
+@pytest.mark.asyncio
+async def test_instructions(client: openai.AsyncOpenAI):
+    response = await client.responses.create(
+        instructions="Finish the answer with QED.",
+        input="What is 13 * 24?",
+    )
+    print(response)
+
+    output_text = response.output[-1].content[0].text
+    assert "312" in output_text
+    assert "QED" in output_text
+
+
+@pytest.mark.asyncio
+async def test_chat(client: openai.AsyncOpenAI):
+    response = await client.responses.create(input=[
+        {
+            "role": "system",
+            "content": "Finish the answer with QED."
+        },
+        {
+            "role": "user",
+            "content": "What is 5 * 3?"
+        },
+        {
+            "role": "assistant",
+            "content": "15. QED."
+        },
+        {
+            "role": "user",
+            "content": "Multiply the result by 2."
+        },
+    ], )
+    print(response)
+
+    output_text = response.output[-1].content[0].text
+    assert "30" in output_text
+    assert "QED" in output_text
+
+
+@pytest.mark.asyncio
+async def test_chat_with_input_type(client: openai.AsyncOpenAI):
+    response = await client.responses.create(input=[
+        {
+            "role": "user",
+            "content": [{
+                "type": "input_text",
+                "text": "Hello!"
+            }],
+        },
+    ], )
+    print(response)
+    assert response.status == "completed"