It is not easy to perform functional tests of the pipline webhook since it requires having an environment configured with GitLab runners in which to run the tests. If you have had the opportunity to test the plugin by activating the webhook for pipelines, please include your comments in this issue indicating if it worked for you or if you found any bugs.
Above all, keep in mind what was commented in the Readme; not all pipelines will be sent from GitLab and processed by OpenProject, only the Merge Request pipelines (see https://gitlab.com/gitlab-org/gitlab/-/issues/345028).
Thank you.