fix: Use correct GraphQL field names in UpdateTransaction mutation

[eshaffer321]

Problem

The Update() method was using incorrect GraphQL field names when updating transactions:

• ❌ Used "categoryId" instead of "category"
• ❌ Used "merchant" instead of "name"

This caused Monarch's API to reject requests with BAD_REQUEST errors, breaking single-category transaction updates since the October 2024 refactoring.

Solution

Fixed using TDD Red-Green-Refactor:

🔴 RED Phase

• Added TestTransactionService_Update_CorrectFieldNames that verifies correct field names
• Test failed as expected, confirming the bug

🟢 GREEN Phase

• Fixed transactions.go lines 155, 158 to use correct field names:
  • "categoryId" → "category"
  • "merchant" → "name"
• Test now passes

🔵 REFACTOR Phase

• Verified all 39 tests pass with no regressions
• Integration tested with real Walmart order - now succeeds

Reference

Field names verified against the Python monarchmoney library (lines 2574-2575), which is the reference implementation.

Verification

• ✅ New test prevents regression
• ✅ All tests pass (39/39)
• ✅ Integration tested with real order
• ✅ Production verified: Order 200013901907426 status changed from failed to success

Files Changed

• pkg/monarch/transactions.go: Fixed field names (2 lines)
• pkg/monarch/transactions_test.go: Added comprehensive test (113 lines)

Impact

Fixes critical bug in single-category transaction updates that has been broken in production.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.18%. Comparing base (05a7f9f) to head (b172145).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (43.18%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   41.80%   43.18%   +1.38%     
==========================================
  Files          15       15              
  Lines        2239     2239              
==========================================
+ Hits          936      967      +31     
+ Misses       1236     1198      -38     
- Partials       67       74       +7     

Flag	Coverage Δ
unittests	43.18% <100.00%> (+1.38%)	⬆️

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

Files with missing lines	Coverage Δ
pkg/monarch/transactions.go	68.08% <100.00%> (+5.71%)	⬆️

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