Rewrite missing_presence_validation to handle polymorphic associations

[gregnavis]

This commits rewrites missing_presence_validation in a more imperative style with extensive commenting to make it clear why certain actions are performed. The new structure made it simple to correctly handle polymorphic associations: if a polymorphic foreign key is NOT NULL then the detector will suggest making the association as required, instead of suggesting adding validators for the _id and _type column separately.

[Copilot]

Pull Request Overview

This PR rewrites the missing_presence_validation detector to handle polymorphic associations correctly by suggesting association-level validation instead of separate validators for foreign key and type columns.

• Rewrote the detector logic in a more imperative style with extensive commenting for better maintainability
• Added proper handling for polymorphic associations to suggest optional: false instead of separate column validators
• Added test coverage for polymorphic association scenarios and configuration options

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
test/setup.rb	Sets default Rails behavior for belongs_to associations to be required by default
test/active_record_doctor/detectors/missing_presence_validation_test.rb	Adds comprehensive test coverage for polymorphic association validation scenarios
lib/active_record_doctor/detectors/missing_presence_validation.rb	Complete rewrite of the detector logic to handle polymorphic associations and improve code clarity