Log refund gateway responses on payments

[BeastNectus]

Summary

• record successful refund gateway responses on the original payment instead of the refund record
• wrap the refund transaction id update and log entry creation in one database transaction
• update the refund spec to assert the payment receives the refund log entry

Closes #4866

Notes

The admin payment page renders @payment.log_entries, so storing the refund response on refund.log_entries meant the gateway response was persisted but not visible from the payment admin view.

Testing

• ruby --disable=gems -c core/app/models/spree/refund.rb
• ruby --disable=gems -c core/spec/models/spree/refund_spec.rb
• git diff --check

I attempted bundle exec rspec core/spec/models/spree/refund_spec.rb, but this Windows environment blocked Ruby's native cgi/escape.so via Application Control policy before the spec suite could boot.

[BeastNectus]

Closing this because #6448 was already open for the same issue before this PR. I'll avoid duplicating the existing review thread.

[BeastNectus]

Reopening because this PR is part of an external take-home assignment submission. I understand there is another open PR for similar work.

[jarednorman]

The transaction approach here doesn't make any sense to me. Closing.