fix: Fix multisign and signfor to check for delegate

[yinyiqian1]

In multisign and signfor operations, an account is not permitted to sign its own transaction.
Similarly, in a delegation transaction, the sfDelegate cannot sign the transaction it is submitting on behalf of another account.

High Level Overview of Change

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)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (93ac1aa) to head (f578e94).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7064     +/-   ##
=========================================
- Coverage     82.1%   82.1%   -0.0%     
=========================================
  Files         1010    1010             
  Lines        76167   76167             
  Branches      7423    7412     -11     
=========================================
- Hits         62514   62506      -8     
- Misses       13653   13661      +8     

Files with missing lines	Coverage Δ
src/libxrpl/protocol/STTx.cpp	88.1% <100.0%> (ø)
src/xrpld/rpc/detail/TransactionSign.cpp	89.1% <100.0%> (ø)

... and 5 files with indirect coverage changes

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[github-actions]

This PR has conflicts, please resolve them in order for the PR to be reviewed.

[godexsoft]

We recently merged a refactor to develop that enables clang-tidy's readability-identifier-naming. Your branch now has heavy conflicts that are largely mechanical. Below is a workflow that aligns your branch's naming with develop before merging, which should minimize the merge conflicts.

One-time setup

If you don't already have clang-tidy working in your env, on macOS:

brew install llvm@21
# Follow brew's hint to put $(brew --prefix llvm@21)/bin on PATH so run-clang-tidy is found.

Workflow on your branch (before merging develop)

1. Grab the new .clang-tidy from develop without pulling anything else. Sync your fork on GitHub first, then:

git remote -v   # should show 'upstream' among others; if not:
# git remote set-url upstream git@github.com:XRPLF/rippled.git
git fetch upstream
git checkout upstream/develop -- .clang-tidy

2. Reconfigure conan/cmake so compile_commands.json is fresh.

3. Apply renames for the files modified in your PR:

git diff --name-only $(git merge-base HEAD upstream/develop) HEAD \
  | grep -E '\.(cpp|h|hpp|ipp)$' \
  | xargs run-clang-tidy -p build -fix -allow-no-checks
# or -p .build, or whatever your build dir is called

4. Build + test, then commit as a single dedicated commit:

cmake --build build -j8
git commit -am "refactor: Align identifier naming with develop"

5. Now merge develop:

git merge upstream/develop

Extra

Run clang-tidy once more after the merge to catch any stragglers introduced from develop's side:

run-clang-tidy -p build -fix -allow-no-checks src tests
# or -p .build, or whatever your build dir is called

[github-actions]

All conflicts have been resolved. Assigned reviewers can now start or resume their review.

[PeterChen13579]

Minor comments

[PeterChen13579]

👍