fix: pass back `usage` when using text mode with VLLM (#5336)

Signed-off-by: Qidong Su <soodoshll@gmail.com>

components/src/dynamo/vllm/handlers.py

@@ -1300,6 +1300,11 @@ class DecodeWorkerHandler(BaseWorkerHandler):
                         "choices": [choice_data],
                     }
 
+                    if output.finish_reason:
+                        chunk["usage"] = BaseWorkerHandler._build_completion_usage(
+                            request_output=res,
+                        )
+
                     yield chunk
 
             except EngineDeadError as e: