Fix column type discovery running too early

[RDIL]

In a test environment, or when creating/seeding the database, there are no tables defined in ActiveRecord. This causes an edge case, where, if any method_missing is called (and it gets to MSL), an error will be thrown. This is because accessing columns_hash or column_names will cause a query to be made that will fail on the Postgres side (as it expects the table to already be present in the database).

For getting caught up on the memes, see the following:

• https://github.com/Genius/Rap-Genius/pull/15200
• Support review apps for rapgenius heroku-buildpack-ruby#14

Additionally, I've fixed another case where column type discovery should not run: abstract classes. This is a common pattern in newer rails apps.

# Column type discovery should NOT be done here
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

  def some_utility_method
    # ...
  end
end

# Column type discovery SHOULD be done here
class User < ApplicationRecord
end

---

Feel free to backport this to Rails 2 - seems pointless since it's On The Way Out (TM) (as it has been for the last decade, but who's counting?!)

Test Plan

Test suite passes on my local branch.
I haven't PR'd in all this work yet, but it allows the tests to run on Ruby 3.

[RDIL]

Closing due to fork: https://github.com/RDIL/modern_searchlogic (it's published to RubyGems now)