ActiveContext: Start using connection model

What does this MR do and why?

This merge request updates the ActiveContext system to use a single active connection model instead of multiple databases. It introduces a new Connection model with an active scope, replaces the Config.databases with Config.connection_model, and modifies the adapter initialization process. The changes simplify the configuration and make it easier to manage active connections. Additionally, the code has been refactored to work with this new connection model, updating related tests and dependencies accordingly.

References

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• Add migration worker (#521128 - closed)

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Before	After

How to set up and validate locally

1. Enable the gem via

   ❯ cat config/initializers/active_context.rb
   # frozen_string_literal: true
   
   ActiveContext.configure do |config|
     config.enabled = true
   end

2. In rails create a new record

   ActiveContext::Config.connection_model.create!(active: true, name: 'pg1', options: {port: 5432, host: 'localhost', username: 'postgres', password: 'password'}, adapter_class: 'ActiveContext::Databases::Postgresql::Adapter', prefix: 'active_context')

3. Create a file ee/db/active_context/migrate/20250123143913_create_merge_requests.rb with

   class CreateMergeRequests < ActiveContext::Migration[1.0]
     milestone '17.9'
   
     def migrate!
       create_collection :merge_requests, number_of_partitions: 3 do |c|
         c.bigint :issue_id, index: true
         c.bigint :namespace_id, index: true
         c.prefix :traversal_ids
         c.vector :embeddings, dimensions: 768
       end
     end
   end

4. Start a postgresql container

   docker run -p 5432:5432 --name pgvector17 -e POSTGRES_PASSWORD=password pgvector/pgvector:pg17

5. Execute the migration

   m = ActiveContext::Migration::Dictionary.migrations.last
   m.new.migrate!

Related to #521128 (closed)