[Instance] Add Managed by column to Duo model selection

What does this MR do and why?

On the instance-level GitLab Duo model selection page, each model dropdown packed a "GitLab-managed model" or "Self-hosted model" subtext and an info icon into the toggle, which was hard to scan.

This MR moves that information into its own Managed by column, between Feature and Model:

• Shows GitLab-managed or Self-hosted for each feature, with a keyboard-focusable tooltip that explains the difference.
• Sortable, like the other columns from !258208 (merged).
• Shown only when the instance has both self-hosted and GitLab-managed models. Offline instances can only use self-hosted models, so the column isn't shown there. This is worked out on the frontend from the feature settings and self-hosted models queries, so the column appears once both queries return.
• The model dropdown toggle text is unchanged; only the subtext and info icon are removed.

Shared components: ModelSelectDropdown drops its now-unused subtext support. The group-level page renders the same as before.

This is the first slice of !260828 (closed), split up per review feedback. The Provider column and model name changes are deferred until improved model data lands, and the Default column needs a separate design for self-hosted models.

References

• Supersedes the "Managed by" part of !260828 (closed) (closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Configure at least one self-hosted model and select it for a feature.
2. Visit Admin > GitLab Duo > Model selection (/admin/gitlab_duo/model_selection).
3. Confirm rows show Self-hosted or GitLab-managed, and hover over or tab to a value to see its tooltip.
4. Select the Managed by header to sort by that column.
5. Pick a different model for a feature and confirm the column updates after it saves.
6. Remove all self-hosted models, or use an instance with only self-hosted models, and reload: the Managed by column is not shown.

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