That being said, you're making my brain click a little bit and this could be converted into a "self contained" toy, with some additional work! The biggest piece of the puzzle is a consistent private key and certificate.
If that is of interest to you, could you create a GitHub Issue as a feature request?
* it seems your package.json is still from an old iteration: https://github.com/saml-to/assume-aws-role-action/blob/main/...
* it was super opaque where this relative import comes from: https://github.com/saml-to/assume-aws-role-action/blob/main/... but after some sniffing around, it seems to be some openapi generation magick https://github.com/saml-to/assume-aws-role-action/blob/main/... against one of your own API endpoints https://github.com/saml-to/assume-aws-role-action/blob/main/... which seems to mean that using this toy is not "self contained" in the way that `sts:AssumeRoleWithWebIdentity` is