fix: Add feePayer logic in txQ

[yinyiqian1]

High Level Overview of Change

Because the delegated transactions are paid by the sfDelegate, but not sfAccount:

• TxQ might reject some transactions that should be queueable by incorrectly counting delegate-paid fees against the principal account.
• It may also undercount the total queued fees owed by a delegate across different accounts, allowing transactions into the queue that the actual fee payer may not be able to cover later.

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

❌ Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.0%. Comparing base (4fec582) to head (1a21b74).
⚠️ Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/xrpld/app/misc/detail/TxQ.cpp	92.7%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7613     +/-   ##
=========================================
- Coverage     82.0%   82.0%   -0.0%     
=========================================
  Files         1007    1007             
  Lines        76890   76925     +35     
  Branches      8971    8979      +8     
=========================================
+ Hits         63049   63059     +10     
- Misses       13832   13857     +25     
  Partials         9       9             

Files with missing lines	Coverage Δ
src/xrpld/app/misc/TxQ.h	100.0% <100.0%> (ø)
src/xrpld/rpc/handlers/account/AccountInfo.cpp	85.5% <100.0%> (+0.1%)	⬆️
src/xrpld/app/misc/detail/TxQ.cpp	96.8% <92.7%> (-0.3%)	⬇️

... and 26 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[depthfirst-app]

🟡 Severity: MEDIUM

The fee payer's test-view balance is reduced only by their aggregate queued fees (totalFee), but ignores their own queued potentialSpend from their personal transaction queue. A fee payer with a large queued payment can appear to have more available balance than they will actually have, allowing delegated transactions into the queue whose fees exceed the payer's real post-spend capacity.
Helpful? Add 👍 / 👎

💡 Fix Suggestion

Suggestion: The fee payer's own queued potentialSpend (from transactions where the fee payer acts as the account) must also be deducted from their test-view balance. To fix this:

1. After computing totalFee (around lines 1059-1072), add a new block that looks up the fee payer's own queued transactions in byAccount_[feePayer] (when feePayer != account) and sums their potentialSpend() into a new variable, e.g. feePayerOwnSpend.

2. Update the balance gate check at line 1124 to also account for the fee payer's own potential spend: change totalFee >= balance to (totalFee + feePayerOwnSpend) >= balance (and similarly for the reserve check).

3. At line 1159, change balance - totalFee to balance - totalFee - feePayerOwnSpend (clamped appropriately to avoid underflow), so the test-view balance reflects both the fees owed as a delegate payer and the fee payer's own queued transaction spend.

This ensures that a fee payer who has queued large payments under their own account cannot appear to have more available balance than they will actually have after those payments execute.

[github-actions]

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