Add require_sandbox to instance and namespace AI settings

What does this MR do and why?

Add require_sandbox to instance and namespace AI settings

Admins need a way to make flows fail when the execution environment sandbox cannot start, instead of running without isolation. Store the setting on ai_settings (instance) and namespace_ai_settings (top-level group). It defaults to false, so behavior is unchanged until the setting is wired up in a follow-up.

#607753 (closed)

Database review notes

Database testing passed db:gitlabcom-database-testing ran both migrations on production clones of the main, ci and sec databases. Each took under 10 ms for the `ALTER TABLE`, with no size change. See the results comment on this MR.

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Related to #607753 (closed)