Instance level model selection migration and rails model

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What is this issue about

We need instance level Model Selection to get this Model Selection access for all scopes e.i GitLab.com, Dedicated, and Self-Managed using the GitLab.com AIG. In this issue we are discussing the migrations and MVC Models changes needed to implement it.

How to setup locally

See this comment

Implementation Proposal

We can implement the Ai::ModelSelection::FeaturesConfigurable concern so we won't have to worry about a lot of the shared logic like validation and such. To implement it you can generate the following model.

gdk rails generate model Ai::ModelSelection::InstanceFeatureSetting feature:integer:index offered_model_ref:text offered_model_name:string

The migration file should look something like this:

class CreateAiNamespaceFeatureSettings < Gitlab::Database::Migration[2.3]
  milestone '18.5'

  def change
    create_table :ai_instance_feature_settings do |t|
      t.timestamps_with_timezone null: false
      t.integer :feature, null: false, limit: 2, index: { unique: true }
      t.text :offered_model_ref, limit: 255, null: true
      t.text :offered_model_name, limit: 255, null: true
    end
  end
end

The model must be at the path ee/app/models/ai/model_selection/instance_feature_setting.rb should look something like this:

# frozen_string_literal: true

module Ai
  module ModelSelection
    class InstanceFeatureSetting < ApplicationRecord
      include ::Ai::ModelSelection::FeaturesConfigurable

      self.table_name = "ai_instance_feature_settings"

      validates :feature, :uniqueness

      scope :non_default, -> { where.not(offered_model_ref: [nil, ""]) }

      def self.find_or_initialize_by_feature(feature)
        return unless ::Feature.enabled?(:ai_model_switching)

        feature_name = get_feature_name(feature)
        find_or_initialize_by(feature: feature_name)
      end

      def model_selection_scope
        :instance
      end

      def set_to_gitlab_default?
        offered_model_ref.blank?
      end
    end
  end
end

Implement some tests the same way it is done in ee/spec/models/ai/model_selection/namespace_feature_setting_spec.rb (There is a lot of shared examples you can reuse)

This should be enough to start development for this.