Simplify creating a new agent with custom reply function

[sonichi]

Why are these changes needed?

When fixing this test error: https://github.com/microsoft/autogen/actions/runs/8349926930/job/22855210369?pr=2078
I tried to simplify the work needed when creating a new agent with custom reply function.

• enrich register_reply and simplify user's code;
• bug fix in gpt assistant agent

Related issue number

#2078 #2005

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

All modified and coverable lines are covered by tests ✅

Project coverage is 60.98%. Comparing base (5b57271) to head (cd967e0).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2095       +/-   ##
===========================================
+ Coverage   36.88%   60.98%   +24.09%     
===========================================
  Files          68       68               
  Lines        7062     7061        -1     
  Branches     1540     1669      +129     
===========================================
+ Hits         2605     4306     +1701     
+ Misses       4228     2365     -1863     
- Partials      229      390      +161     

Flag	Coverage Δ
unittests	60.67% <100.00%> (+23.78%)	⬆️

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