[UI POLISH] Fix Padding on Model Configuration Pages

What does this MR do and why?

Makes padding improvements across the Model Selection and Duo self-hosted pages

Screenshots or screen recordings

Model Configuration/Duo self-hosted

Remove extra padding and line break at top of page:

Before	After

Add padding to end of page:

Before	After

Model Selection

Add padding to end of page:

Before	After

How to set up and validate locally

Set-up

• An active online cloud or legacy, Ultimate license.

To validate Model Selection page (.com):

1. Start GDK in saas mode GITLAB_SIMULATE_SAAS=1 gdk start
2. Enable the ai_model_switching feature flag by visiting http://gdk.test:3000/rails/features/
3. Run the Duo setup rake task GITLAB_SIMULATE_SAAS=1 bundle exec 'rake gitlab:duo:setup' to seed DB with a Duo test group (it will be called gitlab-duo).
4. Visit the test group's model selection page i.e http://gdk.test:3000/groups/gitlab-duo/-/settings/gitlab_duo/model_selection and validate padding looks like "After" screengrabs

To validate Model Configuration/Duo self-hosted pages (self-managed):

1. Start GDK in SM mode GITLAB_SIMULATE_SAAS=0 gdk start
2. Enable the ai_model_switching feature flag by visiting http://gdk.test:3000/rails/features/
3. Run the Duo setup rake task GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup' to set up with an enterprise add-on
4. Visit the model configuration page i.e http://gdk.test:3000/admin/gitlab-duo/self_hosted and validate padding looks like "After" screengrabs

MR acceptance checklist

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