Flush console output after every message.

[afourney]

In some settings, like agbench running via the Docker API, console output is not automatically flushed after newlines (I believe this is only the default for interactive terminals). This make it very difficult to monitor the progress of tasks.

This PR simply restores the flush argument to aprint, and sets it to True after every message or toplevel event. Streaming interactions continue to be buffered.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.09%. Comparing base (3c30d89) to head (5be190c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...gen-agentchat/src/autogen_agentchat/ui/_console.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5415      +/-   ##
==========================================
+ Coverage   78.08%   78.09%   +0.01%     
==========================================
  Files         158      158              
  Lines        9576     9576              
==========================================
+ Hits         7477     7478       +1     
+ Misses       2099     2098       -1     

Flag	Coverage Δ
unittests	78.09% <50.00%> (+0.01%)	⬆️

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