test: add multi-type polymorphic-array spec

The existing array-of-sources test only exercises a single-type array
(`Link.where(source: [cat])`). The fix's value also covers multi-type
arrays (`Link.where(source: [cat, dog])`) — a broader branch of
`type_to_ids_mapping` where the result hash spans multiple keys.

Locks in coverage on both Rails 8.0 (backwards compat) and 8.1.

Suggested by code review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: raymasiclat

spec/polymorphic_integer_type_spec.rb

@@ -136,6 +136,11 @@
       expect(Link.where(source: [source])).to eq [link]
     end
 
+    it "properly finds objects when passing an array of mixed-type sources" do
+      link_for_dog = Link.create(source: dog)
+      expect(Link.where(source: [cat, dog])).to match_array [link, link_for_dog]
+    end
+
     it "properly finds the object with a find_by" do
       expect(Link.find_by(source: source, id: link.id)).to eql link
     end