Avoid loading active record fixtures

[moofkit]

Even when use_active_record = false is set, rspec-rails still included ActiveRecord::TestFixtures and intercepted before and after hooks, attempting to access the databases associated with active record models. This caused issues in non-standard Rails projects where ActiveRecord is present but not using the Rails testing framework directly.

This PR allows projects with ActiveRecord present to cleanly opt-out of fixture-based testing without database hook interference.

Changes

• moved ActiveRecord::TestFixtures inclusion inside the use_active_record? conditional check
• created ActiveRecord.md guide covering fixtures, transactions, and configuration options. Merged Transactions.md to the new document
• updated rails_helper.rb comments to be aligned with updated documentation.