def _prepare_chat for groupchat manager to reset the groupchat

[yenif]

Why are these changes needed?

Dirty messages from a previous chat can cause Tool Calls to raise an exception from the OpenAI API.

Related issue number

https://github.com/microsoft/autogen/actions/runs/7523612676/job/20477791898?pr=1226

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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (260e0cf) 31.98% compared to head (3947fc8) 51.42%.

Files	Patch %	Lines
autogen/agentchat/groupchat.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1254       +/-   ##
===========================================
+ Coverage   31.98%   51.42%   +19.44%     
===========================================
  Files          33       33               
  Lines        4415     4422        +7     
  Branches     1030     1089       +59     
===========================================
+ Hits         1412     2274      +862     
+ Misses       2887     1944      -943     
- Partials      116      204       +88     

Flag	Coverage Δ
unittests	51.35% <90.90%> (+19.42%)	⬆️

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[sonichi]

@joshkyh the merging is blocked by your change request.

[joshkyh]

@joshkyh the merging is blocked by your change request.

Apologies, approved.