feat: add gemini model families, enhance group chat selection for Gemini model and add tests

[ekzhu]

Resolves #5322

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.02%. Comparing base (9af6883) to head (3649818).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...gentchat/teams/_group_chat/_selector_group_chat.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5334   +/-   ##
=======================================
  Coverage   76.02%   76.02%           
=======================================
  Files         157      157           
  Lines        9483     9488    +5     
=======================================
+ Hits         7209     7213    +4     
- Misses       2274     2275    +1     

Flag	Coverage Δ
unittests	76.02% <87.50%> (+<0.01%)	⬆️

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