Add CB API tests on the correct use of max_tokens (#339)

# Description

Add tests to check if the API is generating the correct amount of tokens
when CB is enabled.

## Related Issues

---------

Signed-off-by: Gabriel Marinho <[email protected]>

Author: gmarinho2

tests/e2e/test_spyre_cb.py

@@ -4,8 +4,9 @@
 """
 
 import pytest
-from spyre_util import (generate_spyre_vllm_output, get_chicken_soup_prompts,
-                        get_spyre_model_list)
+from openai import BadRequestError
+from spyre_util import (RemoteOpenAIServer, generate_spyre_vllm_output,
+                        get_chicken_soup_prompts, get_spyre_model_list)
 from vllm import SamplingParams
 
 
@@ -44,6 +45,64 @@ def test_cb_max_tokens(
                                    monkeypatch=monkeypatch)
 
 
+@pytest.mark.cb
+@pytest.mark.parametrize("cb", [True])
+@pytest.mark.parametrize("model", get_spyre_model_list())
+@pytest.mark.parametrize("max_model_len", [256])
+@pytest.mark.parametrize("max_num_seqs", [2])
+@pytest.mark.parametrize(
+    "backend", [pytest.param("eager", marks=pytest.mark.cpu, id="eager")])
+def test__api_cb_rejects_oversized_request(
+    remote_openai_server: RemoteOpenAIServer,
+    model: str,
+    backend: str,
+    cb: bool,
+    max_model_len: int,
+    max_num_seqs: int,
+):
+    """Verify API rejects request that exceed max_model_len with CB enabled"""
+
+    client = remote_openai_server.get_client()
+    overflow_prompt = " ".join(["hi"] * max_model_len)
+    max_tokens = 10
+
+    with pytest.raises(BadRequestError,
+                       match="This model's maximum context length is"):
+        client.completions.create(
+            model=model,
+            prompt=overflow_prompt,
+            max_tokens=max_tokens,
+        )
+
+
+@pytest.mark.cb
+@pytest.mark.parametrize("cb", [True])
+@pytest.mark.parametrize("model", get_spyre_model_list())
+@pytest.mark.parametrize("max_model_len", [256])
+@pytest.mark.parametrize("max_num_seqs", [2])
+@pytest.mark.parametrize(
+    "backend", [pytest.param("eager", marks=pytest.mark.cpu, id="eager")])
+def test_api_cb_generates_correct_max_tokens(
+    remote_openai_server: RemoteOpenAIServer,
+    model: str,
+    backend: str,
+    cb: bool,
+    max_model_len: int,
+    max_num_seqs: int,
+):
+    """Verify API generates the correct numbers of tokens with CB enabled"""
+
+    client = remote_openai_server.get_client()
+    max_tokens = 10
+
+    response = client.completions.create(model=model,
+                                         prompt=get_chicken_soup_prompts(1),
+                                         max_tokens=max_tokens,
+                                         temperature=0)
+
+    assert response.usage.completion_tokens == max_tokens
+
+
 @pytest.mark.cb
 @pytest.mark.spyre
 @pytest.mark.xfail  # TODO: remove once a spyre-base image supports this