handling for string columns in Rails 8

[StephenVNelson]

Description

Fixes a bug in select_for_count (Rails 8.0+) where calling .right on
select_values crashes when the array contains plain strings instead of
Arel nodes.

The Problem

When using Mobility with Rails 8.0+ and calling .count on a query with
selected columns, the code assumes all select_values are Arel::Nodes::As
objects with a .right method. However, select_values can contain a mix of
plain strings (like "id") and Arel nodes, causing:

Article.select(:id).count
# NoMethodError: undefined method `right' for "id":String

The Solution

Added respond_to?(:right) checks before calling .right on select_values
to safely handle both strings and Arel nodes.

Changes Made

• ✅ Added respond_to?(:right) checks in select_for_count method
• ✅ Improved inline documentation explaining the fix
• ✅ Added 3 regression tests covering different scenarios

Test Results

All existing tests pass, plus 3 new tests:

• ✅ Handles count with regular column in select (reproduces original bug)
• ✅ Handles count with translated column in select
• ✅ Handles count without select when mobility is present

# Rails 8.0 with Ruby 3.2.5
15 examples, 0 failures

Related Issues

Closes #678
Related to

1. Fixes for ActiveRecord 8.0 #654 (comment)
2. Fix for Rails 8 query issue breaks select with no order clause #659 (comment)