Preserve authenticated GitLab hosts when glab returns partial failure

GitLab auth probing currently drops authenticated hosts when glab exits nonzero with usable output. Reimplement a host-scoped fallback/cache path with regression coverage. Source PR: https://github.com/stablyai/orca/pull/13052. Contributor credit: @davidia. Owner scope: GitLab provider compatibility; no contributor action requested.

Review the source PR and the GitLab auth probing entry point, focusing on how glab partial failures are handled. Add regression coverage for preserving authenticated hosts and verify that usable output is retained when glab exits nonzero.

Written by the indexing model from the issue text.