Add Google Gemini streaming support

[Sixzero]

This PR adds support for Google's Gemini API to StreamCallbacks.jl.

Changes

• Create stream_gemini.jl with GoogleStream flavor implementation
• Add example script demonstrating Gemini API integration
• Update interface and precompilation files to support Gemini
• Implement chunk extraction and content parsing for Gemini's SSE format

This allows StreamCallbacks to work with Google's server-sent events (SSE) format for streaming LLM responses.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.06122% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.67%. Comparing base (9bc62f3) to head (e686c24).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/stream_gemini.jl	52.08%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   86.81%   81.67%   -5.14%     
==========================================
  Files           6        7       +1     
  Lines         273      322      +49     
==========================================
+ Hits          237      263      +26     
- Misses         36       59      +23     

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[svilupp]

Thank you for the PR!

Could you please add tests for the new functions?
The easiest way is to run your example and provide explicit callback instance, then you can simply dump the captured streamed chunks into clipboard and give them to LLM to generate real-world like test cases.

[svilupp]

@Sixzero could you please add tests for the new functions? Ideally, using real chunks received from Gemini models.