fix: use costco-go NetDiscounts utility, bump dep to v0.3.9

[eshaffer321]

Changes

Replaces the manual discount-netting logic in the Costco provider with the new costco-go v0.3.9 NetDiscounts utility.

Problem

The previous provider.go reimplemented discount matching locally and was still emitting:

[WARN] found orphaned discount with no matching parent item discount_item=379938 parent_item_ref=AAA BATTERY discount_amount=-2.5

on warehouse receipt 21134301000462605131128. The coupon referenced /AAA BATTERY but the parent item was named AA/AAA BATTERY — a partial-description mismatch not handled by the existing exact-match fallback.

Solution

costco-go v0.3.9 ships NetDiscounts([]ReceiptItem) ([]ReceiptItem, []ReceiptItem) which handles this with a three-level matching strategy (item-number → exact-desc → substring/contains). This PR:

• Bumps costco-go from v0.3.6 → v0.3.9
• Replaces ~55 lines of manual netting in provider.go with 2 lines
• Removes the now-unused strings import
• Adds a regression test for the partial-description case

Checklist

• Tests passing (go test ./... -race)
• go.mod / go.sum updated (go mod tidy)
• Regression test added for the failing receipt

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.29%. Comparing base (71053f2) to head (26ceeb8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   56.41%   56.29%   -0.12%     
==========================================
  Files          43       43              
  Lines        4807     4794      -13     
==========================================
- Hits         2712     2699      -13     
  Misses       1931     1931              
  Partials      164      164              

Flag	Coverage Δ
unittests	56.29% <100.00%> (-0.12%)	⬇️

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	39.41% <100.00%> (-4.31%)	⬇️

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