Refactor AssistantAgent on_message_stream

[lspinheiro]

Why are these changes needed?

I'm unsure if everyone will agree, but I started to look into adding new logic and found that refactoring into smaller functions would make it more maintainable.

There is no change in functionality, only a breakdown into smaller methods to make it more modular and improve readability. There is a lot of logic in the method and this refactor breaks it down into context management, llm call and result processing.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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]

Codecov Report

Attention: Patch coverage is 94.11765% with 7 lines in your changes missing coverage. Please review.

Project coverage is 75.64%. Comparing base (a226966) to head (108397c).

Files with missing lines	Patch %	Lines
...t/src/autogen_agentchat/agents/_assistant_agent.py	94.11%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5642      +/-   ##
==========================================
+ Coverage   75.56%   75.64%   +0.08%     
==========================================
  Files         171      171              
  Lines       10483    10542      +59     
==========================================
+ Hits         7921     7975      +54     
- Misses       2562     2567       +5     

Flag	Coverage Δ
unittests	75.64% <94.11%> (+0.08%)	⬆️

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[jackgerrits]

I think a refactor is good, but I think it can hurt understandability of the function. Is it possible to make any of these inner functions static? That way we know exactly what is going in and out of each of them

[ekzhu]

Yeah, there are a lot of things going on in the on_messages_stream now. I think static methods are useful for better visibility.

[lspinheiro]

I updated it to make the inner methods either static or class to make it clearer which state goes into each call. It makes it more verbose because there is a lot of class state reuse (not necessarily being changed) such as the model client and tools being passed internally.