Add e2e tests

> (...) a workflow that calls the locally-referred actions and verifies the result directly via the Flagsmith API. (...)

_Originally posted by @khvn26 in https://github.com/Flagsmith/ci/pull/4#discussion_r2724636039_

Consider this will potentially spam the API with test artefacts. Ideally e2e testing should be pointed at environments with ephemeral data, e.g. a nightly-reset test environment, or a local instance of the API.