fix(refunds): persist log entries when processing refunds

[RamblesShambles]

Summary

When processing a refund via Refund#perform!, the log entry recording the gateway response is built in memory but may not be persisted to the database. This means the admin payment page shows no log entry for the refund operation.

Root cause: log_entries.build creates an in-memory association that relies on Rails' save cascade behavior. The Payment model uses log_entries.create! for the same purpose, which guarantees immediate persistence.

Fix: Change log_entries.build to log_entries.create! in Refund#perform!, matching the established pattern in Payment#record_response.

Closes #4866

How to reproduce

1. Complete a payment on the storefront
2. Go to admin, capture the payment
3. Refund the payment
4. Check for log entries on the payment page
5. Before fix: no log entry for the refund operation
6. After fix: log entry is persisted and queryable

What changed

• core/app/models/spree/refund.rb — Changed log_entries.build to log_entries.create!
• core/spec/models/spree/refund_spec.rb — Added regression test verifying log entry persistence with correct source type

Trade-offs

create! persists the log entry immediately, before the refund itself is saved. If the refund save fails after the gateway credit succeeds, the log entry remains in the DB. This is the same behavior as Payment#record_response and is acceptable because the gateway operation is already irreversible at that point.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (9956911) to head (65a4321).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6456      +/-   ##
==========================================
- Coverage   89.66%   89.45%   -0.22%     
==========================================
  Files         990      919      -71     
  Lines       20793    18981    -1812     
==========================================
- Hits        18645    16980    -1665     
+ Misses       2148     2001     -147     

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