Fix custom model name

[antonpirker]

The model parameter can be a string or an object of the model itself. If it is a model, get the models name from the .model attribute.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.73%. Comparing base (5709b25) to head (5808bec).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4569   +/-   ##
=======================================
  Coverage   80.72%   80.73%           
=======================================
  Files         156      156           
  Lines       16481    16483    +2     
  Branches     2801     2801           
=======================================
+ Hits        13304    13307    +3     
- Misses       2295     2296    +1     
+ Partials      882      880    -2     

Files with missing lines	Coverage Δ
..._sdk/integrations/openai_agents/spans/ai_client.py	83.33% <100.00%> (+0.98%)	⬆️
sentry_sdk/integrations/openai_agents/utils.py	72.80% <100.00%> (+0.24%)	⬆️

... and 4 files with indirect coverage changes