Add support for gitlab v 19

[FBSALOMON]

• Replaces playwright token generation with gitlab-rails runner executed directly inside GitLab test container
• GitLab 19 rejects commit creation requests with an empty actions array unless allow_empty=true is set (GitLab added this requirement in 18.8). Added a nullable AllowEmpty property to CommitCreate and set it in the one test that creates an empty commit
• Fixed GetArchive_QuerySpecifiesAllParameters_AllParametersPassedCorrectly by using the HEAD commit instead of the first commit when combining path=subfolder with a sha, since the subfolder path doesn't exist yet at the first commit. GitLab v18 tolerated this stale path/sha combination (returned an empty folder inside the zip file), but v19 now correctly returns 404 when the requested path doesn't exist at the given revision