clear up deleting a branch with an active session

[dcherian]

It seems I can
(1) repo.writable_session("main")
(2) call repo.delete_branch("main") which deletes the ref using the Storage.
(3) session.branch returns "main" (wrong!)
(4) session.commit raises an hard-to-read ConflictError : icechunk.session.ConflictError: Failed to commit, expected parent: Some("DG260M9H1M6ZVR7330G0"), actual parent: None

Aside from the deletion of "main", we could somehow invalidate the Session.