Move Agentic Chat allow-list under Default model

What does this MR do and why?

The instance-level and group-level GitLab Duo model-selection settings showed the model allow-list ("Available models") for GitLab Duo Agentic Chat as its own table column, next to "Default model". A whole column for that control is heavier than it needs to be.

This MR removes the "Available models" column and moves the control beneath the "Default model" dropdown in the same cell: the allow-list indicator ("XX of YY models allowed" / "All models allowed") sits on the left and a small, tertiary "Configure" button sits on the right, opening the same allow-list modal as before.

The change is scoped to the Agentic Chat section only, at both group and instance level; every other feature section is unchanged. No behavior changes beyond the layout.

References

None.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable instance- or group-level GitLab Duo model selection with the allow-list available for Agentic Chat.
2. Visit the GitLab Duo model settings (group: Settings > GitLab Duo; instance: Admin > GitLab Duo).
3. Find the GitLab Duo Agentic Chat section.
4. Confirm there is no separate Available models column; instead, beneath the Default model dropdown, the allow-list indicator appears on the left with a small tertiary Configure button on the right.
5. Click Configure and confirm the allow-list modal opens as before.

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