[Bugfix][Frontend] Reject guided decoding in multistep mode (#9892)

Signed-off-by: Joe Runde <Joseph.Runde@ibm.com>

tests/entrypoints/openai/test_prompt_validation.py

@@ -35,3 +35,23 @@ async def test_out_of_vocab_token_ids():
                                             prompt=[999999],
                                             max_tokens=5,
                                             temperature=0.0)
+
+
+@pytest.mark.asyncio
+async def test_reject_multistep_with_guided_decoding():
+    model_name = "gpt2"
+    server_args = ["--enforce-eager", "--num-scheduler-steps", "8"]
+    with RemoteOpenAIServer(model_name, server_args) as remote_server:
+        client = remote_server.get_async_client()
+
+        with pytest.raises(openai.BadRequestError,
+                           match=re.compile(
+                               '.*Guided decoding .* multi-step decoding.*')):
+            await client.completions.create(
+                model=model_name,
+                prompt="Hello",
+                max_tokens=5,
+                temperature=0.0,
+                extra_body={"response_format": {
+                    "type": "json_object"
+                }})