Rake task to seed Self-managed with Claude and Codex external agents

What does this MR do and why?

Adds rake task to seed a Self-managed AI Catalog with two specific external agents from GitLab.com:

• Claude Agent by GitLab https://gitlab.com/explore/ai-catalog/agents/2057/
• Codex Agent by GitLab https://gitlab.com/explore/ai-catalog/agents/513/

References

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

How to set up and validate locally

Run the rake task locally:

bundle exec rake gitlab:ai_catalog:seed_external_agents

The task should run:

Seeding AI Catalog with external agents...
Completed successfully!

It has basic idempotency, running it again will fail:

rake aborted!
Error: External agents already seeded
/Users/luke/Sites/gdk/gitlab/ee/lib/gitlab/ai/catalog/third_party_flows/seeder.rb:176:in `run!'
/Users/luke/Sites/gdk/gitlab/ee/lib/gitlab/ai/catalog/third_party_flows/seeder.rb:165:in `run!'
/Users/luke/Sites/gdk/gitlab/ee/lib/tasks/gitlab/ai_catalog.rake:7:in `block (3 levels) in <main>'
Tasks: TOP => gitlab:ai_catalog:seed_external_agents

After running the rake task, visit http://gdk.test:3000/explore/ai-catalog/agents to verify external agents created successfully.

Verify the agents match the ones in the GitLab.com catalog.

MR acceptance checklist

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