[Misc] Add type assertion of request_id for LLMEngine.add_request (#19700)

Signed-off-by: n2ptr <xuzhanchaomail@163.com>

vllm/engine/llm_engine.py

@@ -687,6 +687,10 @@ class LLMEngine:
             >>> # continue the request processing
             >>> ...
         """
+        if not isinstance(request_id, str):
+            raise TypeError(
+                f"request_id must be a string, got {type(request_id)}")
+
         if lora_request is not None and not self.lora_config:
             raise ValueError(f"Got lora_request {lora_request} but LoRA is "
                              "not enabled!")