[Bugfix][Frontend] Guard against bad token ids (#9634)

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

tests/entrypoints/openai/test_prompt_validation.py

@@ -20,3 +20,18 @@ async def test_empty_prompt():
                                             prompt="",
                                             max_tokens=5,
                                             temperature=0.0)
+
+
+@pytest.mark.asyncio
+async def test_out_of_vocab_token_ids():
+    model_name = "gpt2"
+    server_args = ["--enforce-eager"]
+    with RemoteOpenAIServer(model_name, server_args) as remote_server:
+        client = remote_server.get_async_client()
+
+        with pytest.raises(openai.BadRequestError,
+                           match=re.compile('.*out of vocabulary.*')):
+            await client.completions.create(model=model_name,
+                                            prompt=[999999],
+                                            max_tokens=5,
+                                            temperature=0.0)