code optimization

[zhanluxianshen]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.08%. Comparing base (61ba153) to head (cd470ae).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5980   +/-   ##
=======================================
  Coverage   77.08%   77.08%           
=======================================
  Files         197      197           
  Lines       13636    13637    +1     
=======================================
+ Hits        10511    10512    +1     
  Misses       3125     3125           

Flag	Coverage Δ
unittests	77.08% <100.00%> (+<0.01%)	⬆️

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[zhanluxianshen]

Thanks.