Fix PromotionMigrator duplicate codes when promotions share base_code

[NewAlexandria]

Hi — we ran into this during our promotion migration and wanted to see if this would contribute the fix back.

We may be missing something about the intended behavior, so would appreciate a sanity check. Thanks for your time, of course.

I also added promotions/docs/PROMOTION_MIGRATOR_DUPLICATE_CODES_FIX.md with some RCA context, where we saw it, and version info.

@mrshahsuvie FYI

(an llm summary follows)

---

Summary

Fixes PG::UniqueViolation during promotion migration when multiple promotions have batches with the same base_code.

Problem

copy_promotion_codes joins to solidus_promotions_promotion_code_batches on base_code only. If two promotions have batches with the same base code, that join can produce multiple rows per legacy code and attempt duplicate value inserts into solidus_promotions_promotion_codes.

Solution

Constrain the batch join by:

• promotion_id = new_promotion.id
• created_at = spree_promotion_code_batches.created_at

This prevents cross-promotion row multiplication while preserving expected batch matching.

Testing

• Added regression coverage in promotions/spec/lib/solidus_promotions/promotion_migrator_spec.rb for two promotions sharing the same batch base_code.
• Verified migrated code values are inserted exactly once.
• Ran:
  • bundle exec rspec spec/lib/solidus_promotions/promotion_migrator_spec.rb
  • bundle exec rspec (promotions engine suite)

[mamhoff]

I think this documentation should be the commit message, rather than be in the docs folder.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.06%. Comparing base (6477ee8) to head (6280b82).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6422      +/-   ##
==========================================
- Coverage   89.50%   89.06%   -0.44%     
==========================================
  Files         981      785     -196     
  Lines       20476    17297    -3179     
==========================================
- Hits        18327    15406    -2921     
+ Misses       2149     1891     -258     

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[mamhoff]

the conversion to integer in Ruby is unnecessary.