Make foundational agents fixture based
## Summary
Foundational agents should be injected into SQL queries based on fixtures rather than being seeded as database records. This ensures they appear across all Organizations and work correctly on Self-Managed and Dedicated without requiring a rake task or seeding step.
## Background
Work is underway on this via https://gitlab.com/gitlab-org/gitlab/-/merge_requests/232731, which builds on the spike in `knejad-spike-no-foundational-items-in-database` and the existing `FoundationalChatAgent` fixed item model.
The approach uses `ActiveRecord::FixedItemsModel::Model` to inject foundational agents into SQL queries via a VALUES clause, avoiding the need for database records entirely.
## Goals
- Foundational agents appear without any extra seeding on all platforms and organizations
- Pagination and future filtering work without alterations
- Enabling agents in larger groups does not cause scaling issues due to cascading
## References
- Spike MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/226723
- Show page MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/232731
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