Update cache if cost not calculated

[yiranwu0]

Why are these changes needed?

Update cache if cost not calculated, as in #883

Related issue number

Closes #883

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

Comparison is base (31c1664) 25.24% compared to head (cdd854f) 62.70%.

Files	Patch %	Lines
autogen/oai/client.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #903       +/-   ##
===========================================
+ Coverage   25.24%   62.70%   +37.45%     
===========================================
  Files          28       28               
  Lines        3728     3732        +4     
  Branches      848      889       +41     
===========================================
+ Hits          941     2340     +1399     
+ Misses       2710     1145     -1565     
- Partials       77      247      +170     

Flag	Coverage Δ
unittests	62.48% <40.00%> (+37.24%)	⬆️

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