fix: fix the issue where sending assets using Transaction.append_payment_to_contract_op fails when the sender's account is the same as the asset issuer's account.

[overcat]

No description provided.

[overcat]

Thanks to @tamirms for pointing this out. https://github.com/stellar/go/pull/5642#discussion_r2015043692

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.27%. Comparing base (c8045a4) to head (424980b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1029   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files         262      262           
  Lines       14228    14250   +22     
=======================================
+ Hits        13982    14004   +22     
  Misses        246      246           

Flag	Coverage Δ
unittests	98.27% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
stellar_sdk/transaction_builder.py	98.59% <100.00%> (+<0.01%)	⬆️
tests/test_transaction_builder.py	100.00% <100.00%> (ø)

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