Fix STI resolution for models inheriting directly from ActiveRecord::Base (#34)

The base_table_model method previously only stopped traversal when
encountering an abstract superclass. Models whose inheritance chain
reaches ActiveRecord::Base without passing through an abstract class
(like ApplicationRecord) would walk past AR::Base and raise NoMethodError.

Add ActiveRecord::Base as an additional stop condition so STI resolution
works for both inheritance patterns.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>