Raise error when function as llm_config passed to GroupChatManager

[yiranwu0]

1. Raise error when function as llm_config passed to GroupChatManager
2. Add test in test_function_call_groupchat.py.
3. Handling undesired case when selector is returning a function_call instead of str name.

Why are these changes needed?

Related issue number

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• 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 (70f4c1c) 26.73% compared to head (ab97630) 48.59%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #911       +/-   ##
===========================================
+ Coverage   26.73%   48.59%   +21.85%     
===========================================
  Files          28       28               
  Lines        3766     3768        +2     
  Branches      853      896       +43     
===========================================
+ Hits         1007     1831      +824     
+ Misses       2686     1752      -934     
- Partials       73      185      +112     

Flag	Coverage Δ
unittests	48.38% <100.00%> (+21.69%)	⬆️

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