fix: Gateway balance with MPT

[PeterChen13579]

Context of Change

When gateway_balances gets called on an account that is involved in EscrowCreate transaction (with MPT being escrowed), the method returns internal error. This PR fixes this case by excluding the MPT type when totaling escrow amount.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.1%. Comparing base (1eb0fda) to head (934bcc1).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6143   +/-   ##
=======================================
  Coverage     79.1%   79.1%           
=======================================
  Files          839     839           
  Lines        71386   71388    +2     
  Branches      8364    8334   -30     
=======================================
+ Hits         56452   56454    +2     
  Misses       14934   14934           

Files with missing lines	Coverage Δ
src/xrpld/rpc/handlers/GatewayBalances.cpp	88.9% <100.0%> (+0.2%)	⬆️

... and 4 files with indirect coverage changes

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