Remove singleton enforcement from ai_settings

What does this MR do and why?

Removes database-level singleton enforcement from ai_settings so the table shape can eventually support one row per organization:

• Post-deploy migration drops the check_singleton constraint and the index_ai_settings_on_singleton unique index.
• Keeps the singleton column. It will be ignored and dropped in a later iteration.
• Adds validates :organization_id, uniqueness: true, allow_nil: true on Ai::Setting.

SingletonRecord and Ai::Setting.instance are intentionally left unchanged. This means Rails code still preserves the existing singleton behavior in this slice. The finalize iteration replaces SingletonRecord alongside NOT NULL, populate_sharding_key, and the default-organization test fixture work.

Database notes

Index removal

The removed index_ai_settings_on_singleton unique index exists to enforce the old single-row table shape. Keeping it would prevent the future one-row-per-organization shape even after organization_id exists.

The index is on a constant boolean column used only for singleton enforcement. The table remains small, and Rails-level SingletonRecord still preserves current single-row application behavior in this slice.

The replacement uniqueness guarantee is index_ai_settings_on_organization_id, added in !226045 (merged).

Validation safety

The new organization_id uniqueness validation should not break existing records:

• Before this sequence, ai_settings was database-enforced singleton, so duplicate non-null organization_id values cannot already exist.
• The validation allows nil, preserving compatibility while organization_id remains nullable.
• The database unique index on organization_id is already present from !226045 (merged).

References

• Issue: #531356
• Follows merged MR !226045 (merged), which added the nullable organization_id column, unique index, foreign key, and single-row backfill.
• Follows merged MR !243041 (merged), which added the org-scoped lookup API behind a disabled feature flag.

How to set up and validate locally

1. Run the migration:

   bundle exec rails db:migrate

2. Confirm the singleton constraint and index are gone:

   ApplicationRecord.connection.check_constraint_exists?(:ai_settings, 'check_singleton') # => false
   ApplicationRecord.connection.index_exists_by_name?(:ai_settings, 'index_ai_settings_on_singleton') # => false

Validation

• git diff --check
• bundle exec rspec ee/spec/models/ai/setting_spec.rb spec/migrations/db/post_migrate/20260630111100_remove_singleton_from_ai_settings_spec.rb

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