fix: Remove unsupported WithMerchant filter from transaction queries

[eshaffer321]

Summary

Removes the WithMerchant() filter method from transaction queries because the MonarchMoney GraphQL API does not support filtering by merchant name. Using this filter causes a 400 BAD_REQUEST error.

• ❌ Removed: WithMerchant() method from TransactionQueryBuilder interface and implementation
• ✅ Alternative: Users should use Search() method instead, which searches merchants and other text fields
• 📚 Reference: Aligned with Python client implementation which doesn't expose a merchant filter

Problem

The TransactionFilterInput GraphQL type does not include a merchant field. Attempting to use WithMerchant() results in:

Field 'merchant' is not defined by type 'TransactionFilterInput'

According to the Python reference implementation, the supported filters are:

• search, categories, accounts, tags
• hasAttachments, hasNotes, hideFromReports
• isRecurring, isSplit, importedFromMint, syncedFromInstitution
• startDate, endDate

Solution

Before:

transactions, err := client.Transactions.Query().
    WithMerchant("Starbucks").
    Execute(ctx)

After:

transactions, err := client.Transactions.Query().
    Search("Starbucks").  // Searches across merchant names and other text fields
    Execute(ctx)

Changes

• pkg/monarch/interfaces.go: Removed WithMerchant() from interface
• pkg/monarch/transactions.go: Removed implementation, enhanced Search() documentation

Test Plan

• All existing tests pass (54 tests)
• Verified alignment with Python client's supported filters
• Confirmed Search() provides equivalent functionality

Breaking Change

⚠️ This is a breaking change for anyone using WithMerchant(). However:

1. The method never worked correctly (returns API errors)
2. Migration is simple: replace with Search()
3. Functionality is preserved via Search() method

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.82%. Comparing base (eba9f02) to head (e54c9ec).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (44.82%) 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      #18      +/-   ##
==========================================
+ Coverage   44.74%   44.82%   +0.08%     
==========================================
  Files          15       15              
  Lines        1665     1662       -3     
==========================================
  Hits          745      745              
+ Misses        845      842       -3     
  Partials       75       75              

Flag	Coverage Δ
unittests	44.82% <ø> (+0.08%)	⬆️

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	73.50% <ø> (+0.54%)	⬆️

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