fix: Improve json parsing of currency issuers

[kuznetsss]

High Level Overview of Change

This PR adds an early detection of invalid currency issuer.

Context of Change

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)

[Copilot]

Pull request overview

This PR tightens Issue JSON parsing by rejecting reserved/sentinel issuer accounts early, improving detection of invalid non-XRP issuer specifications.

Changes:

• Extend issueFromJson issuer validation to reject noAccount() and xrpAccount() for non-XRP currencies.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (c4c95db) to head (b9e4490).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7110     +/-   ##
=========================================
- Coverage     82.1%   82.1%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        76064   76064             
  Branches      7380    7380             
=========================================
- Hits         62471   62459     -12     
- Misses       13593   13605     +12     

Files with missing lines	Coverage Δ
src/libxrpl/protocol/Issue.cpp	92.7% <100.0%> (ø)

... and 6 files with indirect coverage changes

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