feat: add Gemini model provider implementation

[maximelebastard]

Description

This PR adds native support for Google Gemini models through the Gemini / Google AI Studio API. The implementation includes:

• New GeminiModel class that implements the SDK's Model interface
• Full streaming support with proper event mapping
• Tool use support with correct schema formatting for Gemini API
• Support for text, images, documents, and reasoning content
• Comprehensive error handling (context overflow, throttling)
• 17 unit tests covering all functionality

Uses the official @google/genai SDK.

Related Issues

#330

N/A

Type of Change

New feature

Testing

How have you tested the change?

• I ran npm run check

All tests pass:

• 17 new Gemini-specific unit tests
• All existing tests continue to pass
• 80%+ test coverage maintained
• Type checking, linting, and formatting all pass

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.