Catch token count issue while streaming with customized models

[BeibinLi]

If llama, llava, phi, or some other models are used for streaming (with stream=True), the current design would crash after fetching the response.

A warning is enough in this case, just like the non-streaming use cases.

Why are these changes needed?

Related issue number

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: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 21.29%. Comparing base (6aaa238) to head (7d1a110).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
autogen/oai/client.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3241       +/-   ##
===========================================
- Coverage   33.24%   21.29%   -11.95%     
===========================================
  Files          99       99               
  Lines       11016    11020        +4     
  Branches     2365     2537      +172     
===========================================
- Hits         3662     2347     -1315     
- Misses       7026     8507     +1481     
+ Partials      328      166      -162     

Flag	Coverage Δ
unittests	21.26% <0.00%> (-11.99%)	⬇️

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