[Mohith26]

Fixes #69.

On Rails 7.1+, CollectionAssociation#merge_target_lists copies reloaded rows onto records the caller already holds with mem_record._write_attribute(name, record[name]). For polymorphic integer type columns the gem's [] returns the mapped class name, and _write_attribute skips the gem's []=, so the string goes straight into the integer column and is cast to 0. The database row is fine, but the in-memory record now points at whatever type is mapped to 0 (or nothing).

This adds a _write_attribute override next to [] / []=: when the attribute is one of the model's polymorphic foreign types and the value is a String, it's mapped back through <foreign_type>_mapping before calling super. Integer values (what the generated setters pass) are untouched, and an unknown string falls through unchanged.

Tests:

• New spec "When a has_many reload merges into records the caller still holds" builds two links through cat.source_links.new(target: ...), saves them, triggers the reload with .length, and checks the held records keep target_type 2 / "Drink". It fails on master (got: 0) and passes with the fix.
• bundle exec rspec: 43 examples, 0 failures on ActiveRecord 8.1.4 and on 7.2.4 (Ruby 4.0.6 locally).
• The reproduction script from the issue prints FAIL on master and PASS with this change, on both 8.1.4 and 7.2.4.

I didn't run the 7.0 gemfile locally; that version doesn't go through _write_attribute in merge_target_lists, so the override should be a no-op there. Added a CHANGELOG entry under Unreleased and left the version alone.