Document that offline licenses have no GitLab-managed models

What does this MR do?

An offline cloud license has no connection to the GitLab.com AI Gateway, so Ai::ModelSelection::FetchModelDefinitionsService returns no GitLab-managed models and the model selection list is empty, with no error. The docs describe choosing GitLab-managed models in a hybrid self-hosted setup without that carve-out, and the only offline note on the model selection page reads as an add-on prerequisite for changing a model, which implies models are selectable. This states the behavior on the model selection page and points to it from the two self-hosted pages that describe the choice.

I reproduced this on omnibus 19.3.1-ee with an offline cloud license: the service returns success with a nil payload, the instance GraphQL model lists come back empty with no errors key, and the admin model picker renders empty with no message. The gate itself is deliberate and commented in ee/app/services/ai/feature_setting_selection_service.rb, so the gap is in the documentation.

Related issues

None. The behavior is intentional; this MR documents it.

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AI-Generated Content Disclosure: This MR was prepared with assistance from Claude Code. I reviewed the output for correctness, verified each claim against the source and against a reproduction on omnibus 19.3.1-ee, and checked it against the documentation style guide.