refactor(forge): GitLab builds a RestClient it only uses for pagination #1288

Raised in the review of #1287. RestClient::gitlab builds a project URL that is never read and hands GitLab a client exposing GitHub/Gitea-shaped release and comment calls (/issues/..., body), which would hit the wrong GitLab endpoints if anyone called them. Move the pagination loop into a free function all three forges share and keep RestClient for GitHub and Gitea.