fix: avoid double-counting tax in Amazon splits

[eshaffer321]

Summary

• Prevent the Amazon splitter from adding order-level tax on top of charge-allocated item prices.
• Add a regression using the reported multi-category order values and document the proportional allocation limitation.

Verification

• go test -race ./...
• make pre-commit-local
• Amazon dry run was attempted but could not fetch orders because the local erick profile is missing essential cookies.

Related issue: #100

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.48%. Comparing base (0d7a6aa) to head (a2b7468).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   65.37%   65.48%   +0.10%     
==========================================
  Files          51       51              
  Lines        7036     7038       +2     
==========================================
+ Hits         4600     4609       +9     
+ Misses       2094     2087       -7     
  Partials      342      342              

Flag	Coverage Δ
unittests	65.48% <100.00%> (+0.10%)	⬆️

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	73.40% <100.00%> (+2.68%)	⬆️

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