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

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

vllm/v1/engine/llm_engine.py

@@ -192,6 +192,11 @@ class LLMEngine:
         prompt_adapter_request: Optional[PromptAdapterRequest] = None,
         priority: int = 0,
     ) -> None:
+        # Validate the request_id type.
+        if not isinstance(request_id, str):
+            raise TypeError(
+                f"request_id must be a string, got {type(request_id)}")
+
         # Process raw inputs into the request.
         prompt_str, request = self.processor.process_inputs(
             request_id, prompt, params, arrival_time, lora_request,