Fix: deserialize model_context in AssistantAgent and SocietyOfMindAgent and CodeExecutorAgent

[SongChiYoung]

Why are these changes needed?

This PR fixes a bug where model_context was either ignored or explicitly set to None during agent deserialization (_from_config) in:

• AssistantAgent: model_context was serialized but not restored.
• SocietyOfMindAgent: model_context was neither serialized nor restored.
• CodeExecutorAgent: model_context was serialized but not restored.

As a result, restoring an agent from its config silently dropped runtime context settings, potentially affecting agent behavior.

This patch:

• Adds proper serialization/deserialization of model_context using .dump_component() and load_component(...).
• Ensures round-trip consistency when using declarative agent configs.

Related issue number

Closes #6336

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.

[ekzhu]

Good catch!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.26%. Comparing base (8a97292) to head (5283a77).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6337      +/-   ##
==========================================
+ Coverage   78.23%   78.26%   +0.02%     
==========================================
  Files         217      217              
  Lines       15666    15669       +3     
==========================================
+ Hits        12257    12263       +6     
+ Misses       3409     3406       -3     

Flag	Coverage Δ
unittests	78.26% <100.00%> (+0.02%)	⬆️

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