fix: add state management for oai assistant

[lspinheiro]

Why are these changes needed?

To allow serialization of OAI Assistant Agent.

Related issue number

Closes #5130

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.73%. Comparing base (5df5bde) to head (cb11492).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5352      +/-   ##
==========================================
+ Coverage   76.02%   77.73%   +1.70%     
==========================================
  Files         157      157              
  Lines        9491     9509      +18     
==========================================
+ Hits         7216     7392     +176     
+ Misses       2275     2117     -158     

Flag	Coverage Δ
unittests	77.73% <100.00%> (+1.70%)	⬆️

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