fix: Check network ID in transactionSignFor

[kuznetsss]

High Level Overview of Change

This PR adds NetworkID field validation to sign_for RPC.

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

Adds NetworkID validation to the sign_for RPC path so callers can’t generate multi-signature payloads for the wrong network, and adjusts Expected::error() to support moving errors out of temporaries.

Changes:

• Add checkNetworkID() validation and invoke it from transactionSignFor().
• Add ref-qualified Expected::error() overloads, including an rvalue overload to enable move-out of error values.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/xrpld/rpc/detail/TransactionSign.cpp	Introduces and applies NetworkID validation for sign_for requests.
include/xrpl/basics/Expected.h	Adds ref-qualified error() overloads (including rvalue) to support moving error payloads.

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[codecov]

Codecov Report

❌ Patch coverage is 52.63158% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.1%. Comparing base (6340c98) to head (b39c9cc).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/xrpld/rpc/detail/TransactionSign.cpp	46.2%	7 Missing ⚠️
include/xrpl/basics/Expected.h	66.7%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7102     +/-   ##
=========================================
- Coverage     82.2%   82.1%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        76147   76162     +15     
  Branches      7377    7381      +4     
=========================================
- Hits         62556   62555      -1     
- Misses       13591   13607     +16     

Files with missing lines	Coverage Δ
include/xrpl/basics/Expected.h	96.2% <66.7%> (-3.8%)	⬇️
src/xrpld/rpc/detail/TransactionSign.cpp	89.1% <46.2%> (-1.0%)	⬇️

... and 2 files with indirect coverage changes

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