What is the ECCN number for swagger-codegen-cli-3.0.31.jar

That would be best to ask the OpenAPI tooling group; we do not maintain that tool nor have we any sort of ownership of its classification and I don't think anyone on the forum here can give a correct answer.

They might have info on their site here: swagger.io/tools/swagger-codegen/

Or in their repo here: github.com/swagger-api/swagger-codegen