Did you start the plumber API you are trying to reach using this port? I do not see any change in the repo.
Also try setting your host in pr$run. Connection refused is different from cannot resolve host name. It makes me wonder...
pr$run(host = "0.0.0.0", port = port)
Other things I'm thinking. Maybe the CI tool (github actions) is not letting you open port on the machine to avoid exploits?
Do the test work locally?