Add support for Gemini 2.5 flash stable

[DavidSchmidt00]

Why are these changes needed?

As Gemini 2.5 Flash was released as stable the model infos should be changed accordingly.

See https://ai.google.dev/gemini-api/docs/models?hl=de#gemini-2.5-flash

Related issue number

No issue

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.

[DavidSchmidt00]

@microsoft-github-policy-service agree

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.71%. Comparing base (89927ca) to head (f7ddf1e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6692   +/-   ##
=======================================
  Coverage   79.71%   79.71%           
=======================================
  Files         232      232           
  Lines       17323    17323           
=======================================
  Hits        13809    13809           
  Misses       3514     3514           

Flag	Coverage Δ
unittests	79.71% <ø> (ø)

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