fix: preserve Amazon authentication cookies

[eshaffer321]

Summary

• fix Amazon setup validation so a fresh browser login reaches the real auth check instead of parsing an empty temporary JSON file
• prevent failed Amazon health checks from overwriting saved account cookies with sign-in response cookies
• continue persisting legitimate cookie refreshes after successful order and order-detail fetches
• add regression coverage and document the incident

Root cause

The setup validator created an empty temporary file, then passed that existing file to amazon-go, whose cookie store tried to parse it as JSON. Separately, the provider used amazon-go's default auto-save behavior during health checks, so Amazon's sign-in response could replace saved session cookies before the health check returned an authentication error.

Impact

Users can complete itemize amazon setup -account <name> normally, and an authentication failure is now read-only. Successful Amazon operations still persist refreshed cookies.

Validation

• go test ./...
• go test ./... -race
• go build -o <temp>/itemize ./cmd/itemize/
• pre-commit golangci-lint, go vet, and full tests
• real Amazon browser-profile import using a copied profile and temporary cookie destination: auth check passed
• isolated amazon -dry-run -days 14 -max 1 fetched one order successfully and made no Monarch writes

The branch was fetched and rebased onto origin/main immediately before push.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.10%. Comparing base (b01acb1) to head (e786221).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   61.81%   62.10%   +0.28%     
==========================================
  Files          46       46              
  Lines        6021     6040      +19     
==========================================
+ Hits         3722     3751      +29     
+ Misses       2046     2035      -11     
- Partials      253      254       +1     

Flag	Coverage Δ
unittests	62.10% <100.00%> (+0.28%)	⬆️

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

Files with missing lines	Coverage Δ
internal/adapters/providers/amazon/provider.go	85.39% <100.00%> (+6.32%)	⬆️

... and 1 file with indirect coverage changes

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