[Core / Bugfix] Add Mechanism for Endpoint Rejection (#4762)

Signed-off-by: Alex Brooks <albrooks@redhat.com>

tests/e2e/online_serving/test_qwen3_omni.py

@@ -226,3 +226,27 @@ def test_thinker_prefix_caching_audio_output(omni_server, openai_client) -> None
     }
 
     _run_prefix_cache_check(openai_client, request_config)
+
+
+@pytest.mark.advanced_model
+@pytest.mark.core_model
+@pytest.mark.omni
+@hardware_test(res={"cuda": "H100", "rocm": "MI325"}, num_cards=2)
+@pytest.mark.parametrize("omni_server", test_params, indirect=True)
+def test_completions_rejected_for_thinker_talker(omni_server, openai_client) -> None:
+    """Ensure Thinker-talker models reject /v1/completions; we do this because the
+    thinker-talker handoff implementations currently use ChatML <|im_start|> and
+    <|im_end|> markers to segment the input sequence; when we don't have them,
+    the talker does not get any embeddings, which breaks the server.
+    """
+    responses = openai_client.send_completions_http_request(
+        {
+            "json": {
+                "model": omni_server.model,
+                "prompt": "Hello, how are you?",
+                "max_tokens": 10,
+            },
+        },
+        err_code=400,
+    )
+    assert not responses[0].success