Support Rails 8.1 in PolymorphicArrayValueExtension

[raymasiclat]

Rails 8.1 changed ActiveRecord::PredicateBuilder::PolymorphicArrayValue#initialize from (associated_table, values) to (reflection, values), dropping the @associated_table ivar that this gem reads via @associated_table.send(:reflection) in type_to_ids_mapping. Without an update, every polymorphic-array predicate query (Model.where(thing: [a, b])) raises NoMethodError: undefined method 'reflection' for nil on Rails 8.1.

Fix

Read @reflection when set (Rails 8.1+), fall back to @associated_table.send(:reflection) for older Rails versions. The rest of type_to_ids_mapping runs unchanged.

 def type_to_ids_mapping
-  association = @associated_table.send(:reflection)
+  association = if instance_variable_defined?(:@reflection) && @reflection
+    @reflection
+  else
+    @associated_table.send(:reflection)
+  end

Also in this PR

• Adds gemfiles/Gemfile.rails-8.1-stable to the CI matrix (with Ruby 3.0/3.1 exclusions — Rails 8.1 requires 3.2+)
• Bumps version to 3.5.0
• CHANGELOG entry

Verification

The existing spec at spec/polymorphic_integer_type_spec.rb:135 ("properly finds the object when passing an array of sources") exercises exactly this code path:

Link.where(source: [source])

Verified locally:

• Rails 8.0: 41/41 passing (backwards compat preserved)
• Rails 8.1: 41/41 passing (fix works)
• Rails 8.1 without this fix: 1 failure — proves the existing test catches the regression and that this fix resolves it

Downstream impact

Several Clio repos currently need (or are about to need) in-app monkey patches for this on Rails 8.1 — clio/accounting (PR #15318), and likely clio/themis, clio/grow. Publishing 3.5.0 lets each of those bump the gem dep and prevent the need for in-app patches.

🤖 Generated with Claude Code

[ChrisRoversClio]

manual and automated review. CI green. tests look good. LGTM