Support batch updates for managed models

What does this MR do and why?

Support batch updates for managed models

Changelog: added

EE: true

Adds Apply to all support for GitLab-managed models in the instance-level GitLab Duo model selection settings.

The batch updater now:

• Supports managed, vendored, and self-hosted model configurations.
• Applies an explicitly selected managed model only when every feature supports it.
• Resets each feature to its own default model when Default is selected.
• Respects instance model selection permissions.
• Shows the unassigned-model warning only when no usable default exists.

This fixes the missing Apply to all action observed on Self-Managed 19.2.0-jh when using managed models.

References

to fix this issue: #607810 (closed)

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Before	After

How to set up and validate locally

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