Fix bug in async group chat with func/tool call

[sonichi]

Why are these changes needed?

The async group chat doesn't preprocess tool call message the same way as the sync. This PR fixes that by extracting the common code out. This PR also adds tests for the case where there are >2 agents in the group chat, with 1 agent for tool proposer, 1 agent for execution, and 1 agent not knowing the tool definition.

This PR also fixes a bug in test code introduced in #1227 which will fail the openai test and block other PRs.

Related issue number

Close #1238

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: 9 lines in your changes are missing coverage. Please review.

Comparison is base (42674c5) 31.45% compared to head (db2d953) 41.46%.

Files	Patch %	Lines
autogen/agentchat/groupchat.py	66.66%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1243       +/-   ##
===========================================
+ Coverage   31.45%   41.46%   +10.00%     
===========================================
  Files          32       32               
  Lines        4393     4389        -4     
  Branches     1026     1079       +53     
===========================================
+ Hits         1382     1820      +438     
+ Misses       2902     2427      -475     
- Partials      109      142       +33     

Flag	Coverage Δ
unittests	41.39% <66.66%> (+9.98%)	⬆️

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