[Bugfix][Frontend] validate arg priority in frontend LLM class before add request (#27596)

Signed-off-by: Junpu Fan <junpufan@gmail.com>

vllm/entrypoints/llm.py

@@ -1565,6 +1565,12 @@ class LLM:
             raise ValueError(
                 "The lengths of prompts and lora_request must be the same."
             )
+        if priority is not None and len(priority) != num_requests:
+            raise ValueError(
+                "The lengths of prompts "
+                f"({num_requests}) and priority ({len(priority)}) "
+                "must be the same."
+            )
 
         for sp in params if isinstance(params, Sequence) else (params,):
             if isinstance(sp, SamplingParams):