Add affected_rows to ActiveRecord::Result

[aidanharan]

Add affected_rows to ActiveRecord::Result. See rails/rails#55060

Also, removed logic from cast_result(result) as the results have already been converted to ActiveRecord::Result by perform_query.

Fixes https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/actions/runs/15134938986/job/42544441561

  1) Failure:
ActiveRecord::AdapterTest#test_0003_#exec_query queries return an ActiveRecord::Result with affected rows [/usr/local/bundle/bundler/gems/rails-d1baada0da5a/activerecord/test/cases/adapter_test.rb:158]:
Expected: 2
  Actual: nil