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 79.8%. Comparing base (3cd1e3d) to head (484cf80).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6143     +/-   ##
=========================================
- Coverage     79.8%   79.8%   -0.0%     
=========================================
  Files          858     858             
  Lines        67764   67766      +2     
  Branches      7575    7555     -20     
=========================================
- Hits         54084   54076      -8     
- Misses       13680   13690     +10     

Files with missing lines	Coverage Δ
src/xrpld/rpc/handlers/GatewayBalances.cpp	89.4% <100.0%> (+0.2%)	⬆️

... and 4 files with indirect coverage changes

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[godexsoft]

I'm worried that this is not a fix but just ignoring the problem. If it's the correct way then it is fine.

[shawnxie999]

@godexsoft this is the correct fix. Gateway balance is an RPC that's only applicable to IOUs, so we need to skip whenever we see a MPT type.

[godexsoft]