[Frontend] add json_schema support from OpenAI protocol (#7654)

tests/entrypoints/openai/test_chat.py

@@ -837,6 +837,39 @@ async def test_response_format_json_object(client: openai.AsyncOpenAI):
         assert loaded == {"result": 2}, loaded
 
 
+@pytest.mark.asyncio
+async def test_response_format_json_schema(client: openai.AsyncOpenAI):
+    for _ in range(2):
+        resp = await client.chat.completions.create(
+            model=MODEL_NAME,
+            messages=[{
+                "role":
+                "user",
+                "content": ('what is 1+1? please respond with a JSON object, '
+                            'the format is {"result": 2}')
+            }],
+            response_format={
+                "type": "json_schema",
+                "json_schema": {
+                    "name": "foo_test",
+                    "schema": {
+                        "type": "object",
+                        "properties": {
+                            "result": {
+                                "type": "integer"
+                            },
+                        },
+                    },
+                }
+            })
+
+        content = resp.choices[0].message.content
+        assert content is not None
+
+        loaded = json.loads(content)
+        assert loaded == {"result": 2}, loaded
+
+
 @pytest.mark.asyncio
 async def test_extra_fields(client: openai.AsyncOpenAI):
     with pytest.raises(BadRequestError) as exc_info: