Fix mode dump with different pydantic version

[yiranwu0]

Why are these changes needed?

Closes #1058

Related issue number

Closes #1058

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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (5b420f1) 30.21% compared to head (4b19b85) 40.66%.

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1063       +/-   ##
===========================================
+ Coverage   30.21%   40.66%   +10.44%     
===========================================
  Files          30       30               
  Lines        3988     3989        +1     
  Branches      899      948       +49     
===========================================
+ Hits         1205     1622      +417     
+ Misses       2704     2236      -468     
- Partials       79      131       +52     

Flag	Coverage Δ
unittests	40.61% <50.00%> (+10.44%)	⬆️

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