fix(mempool): prevent memory leaks in PriorityNonceMempool Remove

[songgaoye]

Description

clean up empty data structures in the Remove function to prevent memory leaks:

• delete sender indices when the last transaction for a sender is removed
• delete priority count entries when they reach zero

Closes: #XXXX

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.85%. Comparing base (0886f83) to head (b589a94).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25858      +/-   ##
==========================================
- Coverage   64.87%   64.85%   -0.03%     
==========================================
  Files         890      874      -16     
  Lines       58581    57603     -978     
==========================================
- Hits        38006    37358     -648     
+ Misses      20575    20245     -330     

Files with missing lines	Coverage Δ
types/mempool/priority_nonce.go	90.70% <100.00%> (-1.64%)	⬇️

... and 19 files with indirect coverage changes

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[aljo242]

Can we test this?

Is there a bug or behavior failure this is addressing?

[songgaoye]

add unit test TestPriorityNonceMempool_CleanupOnRemove exerecise the cleanup added in PriorityNonceMempool.Remove.

Before the fix, removing the last tx for a sender left an empty skip list in senderIndices and a zero entry in priorityCounts. Those stale structures accumulated over time, effectively leaking memory and preventing IsEmpty from ever succeeding after deletes.

[aljo242]

@songgaoye lgtm - conflicts

[songgaoye]

update. please take a look at @aljo242 .