fix: categorize Walmart refund items

[eshaffer321]

Summary

• match Walmart ledger refunds to positive Monarch credits
• extract Walmart item-level returnId metadata and categorize only the refunded item
• leave ambiguous or multi-refund cases untouched rather than splitting a refund across the full cart
• retain refund transaction audit records

Verification

• go test ./...
• go test ./... -race
• ./itemize walmart -dry-run -force -days 14 -order-id 200014872726122 -verbose
  • matched the $5.58 credit for order 200014872726122
  • identified the Chobani Coffee Creamer refund item
  • dry-ran a single-category update with no Monarch writes

[codecov]

Codecov Report

❌ Patch coverage is 50.71770% with 103 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.09%. Comparing base (2ea9f71) to head (00f6d58).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/application/sync/handlers/walmart.go	51.57%	63 Missing and 14 partials ⚠️
internal/application/sync/recording.go	0.00%	14 Missing ⚠️
internal/adapters/providers/walmart/order.go	66.66%	10 Missing and 2 partials ⚠️

❌ Your patch check has failed because the patch coverage (50.71%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
- Coverage   61.23%   61.09%   -0.14%     
==========================================
  Files          46       46              
  Lines        5837     6022     +185     
==========================================
+ Hits         3574     3679     +105     
- Misses       2027     2092      +65     
- Partials      236      251      +15     

Flag	Coverage Δ
unittests	61.09% <50.71%> (-0.14%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
internal/application/sync/handlers/amazon.go	65.28% <ø> (ø)
internal/adapters/providers/walmart/order.go	70.00% <66.66%> (+6.58%)	⬆️
internal/application/sync/recording.go	1.19% <0.00%> (-0.11%)	⬇️
internal/application/sync/handlers/walmart.go	59.25% <51.57%> (-2.09%)	⬇️

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