Enable GoogleGenAI extension

[svilupp]

Supersedes #102

• Re-enabled the GoogleGenAI extension since GoogleGenAI.jl is now officially registered. You can use aigenerate by setting the model to gemini and providing the GOOGLE_API_KEY environment variable.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.38%. Comparing base (38474f8) to head (99420cf).

Files	Patch %	Lines
ext/GoogleGenAIPromptingToolsExt.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   92.77%   93.38%   +0.60%     
==========================================
  Files          40       40              
  Lines        2838     2841       +3     
==========================================
+ Hits         2633     2653      +20     
+ Misses        205      188      -17     

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