fix: Block delegate tx from being queued

[yinyiqian1]

High Level Overview of Change

delegate tx is disallowed to enter txQ

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 82.0%. Comparing base (0711a7b) to head (9bfbe8e).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #7640     +/-   ##
=========================================
- Coverage     82.0%   82.0%   -0.0%     
=========================================
  Files         1007    1007             
  Lines        76890   76890             
  Branches      8976    8972      -4     
=========================================
- Hits         63049   63040      -9     
- Misses       13832   13841      +9     
  Partials         9       9             

Files with missing lines	Coverage Δ
src/xrpld/app/misc/detail/TxQ.cpp	97.1% <100.0%> (+<0.1%)	⬆️

... and 12 files with indirect coverage changes

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[ximinez]

Other than adding the test @shawnxie999 asked for, this looks good.

[Copilot]

Pull request overview

This PR prevents delegated transactions (those with sfDelegate present) from being admitted into the transaction queue (TxQ), while still allowing them to apply directly to the open ledger when they pay the required open-ledger fee.

Changes:

• Add a TxQ eligibility check that rejects any transaction containing sfDelegate from being queued (telCAN_NOT_QUEUE).
• Add a unit test that verifies delegated transactions cannot be queued, but can still succeed when submitted with the open-ledger fee.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/xrpld/app/misc/detail/TxQ.cpp	Rejects delegated transactions in TxQ::canBeHeld() so they can’t enter TxQ.
src/test/app/TxQ_test.cpp	Adds coverage ensuring delegated txs are rejected from TxQ but still succeed with sufficient fee.

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