do model check properly

[sonichi]

Why are these changes needed?

Fix test error introduced by #1654. Move model check from client to ConversableAgent, and modified corresponding tests.

Related issue number

#1522

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 (d677b47) 39.50% compared to head (1abb998) 61.98%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1686       +/-   ##
===========================================
+ Coverage   39.50%   61.98%   +22.48%     
===========================================
  Files          56       56               
  Lines        5994     5985        -9     
  Branches     1337     1450      +113     
===========================================
+ Hits         2368     3710     +1342     
+ Misses       3431     1944     -1487     
- Partials      195      331      +136     

Flag	Coverage Δ
unittests	61.75% <100.00%> (+22.24%)	⬆️

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[gunnarku]

Mea culpa. Thank you for fixing my mess. I owe you one.