fix: apply Costco discounts referenced by description instead of item number

[eshaffer321]

Summary

• Costco sometimes references a discount's parent item by description (e.g. /AAA BATTERY) rather than by item number (e.g. /1234567)
• Previously these were logged as orphaned and the -$2.50 discount was dropped, leaving the item at full price
• Fix adds a descMap keyed by uppercased description as a fallback when the item-number lookup misses

Test plan

• New test: discount references parent by description instead of item number — red before fix, green after
• All existing discount-netting tests still pass
• go test ./... -race clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.41%. Comparing base (9cdcc9f) to head (997f79b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   56.36%   56.41%   +0.05%     
==========================================
  Files          43       43              
  Lines        4801     4807       +6     
==========================================
+ Hits         2706     2712       +6     
  Misses       1931     1931              
  Partials      164      164              

Flag	Coverage Δ
unittests	56.41% <100.00%> (+0.05%)	⬆️

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

Files with missing lines	Coverage Δ
internal/adapters/providers/costco/provider.go	43.71% <100.00%> (+1.90%)	⬆️

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