change code_execution_config default

[sonichi]

Why are these changes needed?

Changing this default because the default of last_n_messages is changed to "auto" now.

Related issue number

#1356

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b0c1f8d) 33.12% compared to head (9d7eca6) 53.51%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1518       +/-   ##
===========================================
+ Coverage   33.12%   53.51%   +20.39%     
===========================================
  Files          42       42               
  Lines        5051     5051               
  Branches     1157     1224       +67     
===========================================
+ Hits         1673     2703     +1030     
+ Misses       3250     2114     -1136     
- Partials      128      234      +106     

Flag	Coverage Δ
unittests	53.43% <100.00%> (+20.35%)	⬆️

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