Backport of rake task to seed AI Catalogs with external agents

What does this MR do and why?

Rake task to seed AI Catalogs with external agents

This adds a rake task that self-managed GitLab customers can run to populate their databases with Claude Code and Codex external agents for their AI Catalog.

The ability to use BYOK External Agents (specifically Claude Code and Codex) was required for 18.8, but wasn't included. This is part of a few merge requests that we agreed as a one-off exception to the "bug fixes only" policy for backports (See more in the #external-agents-ga Slack channel (Internal, good for 90 days).

https://gitlab.com/gitlab-org/gitlab/-/work_items/586347

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.
  • Not a bug fix, but an agreed exception to the process
• 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, reach out to a Software Engineer in Test in #s_developer_experience.

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.