Backport 'Restore AI infrastructure settings after 19.2 upgrades' to 19-2-stable-ee

What does this MR do and why?

Backport of !246616 (merged) to the 19-2-stable-ee stable branch.

Restores five AI infrastructure settings for Self-Managed instances affected by an upgrade to GitLab 19.2.

The regression was caused by the interaction of two migrations:

• !226045 (merged) adds ai_settings.organization_id in a regular migration, then sets the existing singleton row to the default organization in a post-deploy migration.
• !243574 (merged) copies the five settings from the default organization's ai_settings row to application_settings in a regular migration.

On Self-Managed upgrades, within milestone 19.2, all regular migrations run before post-deploy migrations. The copy therefore ran while the legacy row still had a NULL organization_id, found no default-organization row, and left the new columns at their defaults.

This post-deploy migration runs after the organization backfill and restores each setting from the preserved legacy row only when its destination still has the default introduced by the failed copy.

Related work item: #606458

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.