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

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

tests/entrypoints/llm/test_prompt_validation.py

@@ -4,6 +4,12 @@ from vllm import LLM
 
 
 def test_empty_prompt():
-    llm = LLM(model="gpt2")
+    llm = LLM(model="gpt2", enforce_eager=True)
     with pytest.raises(ValueError, match='Prompt cannot be empty'):
         llm.generate([""])
+
+
+def test_out_of_vocab_token():
+    llm = LLM(model="gpt2", enforce_eager=True)
+    with pytest.raises(ValueError, match='out of vocabulary'):
+        llm.generate({"prompt_token_ids": [999999]})