feat: add custom Gemini endpoint support

- Add GEMINI_BASE_URL configuration option in .env.example
- Implement custom endpoint support in GeminiModelProvider using HttpOptions
- Update registry to pass base_url parameter to Gemini provider
- Maintain backward compatibility - uses default Google endpoint when not configured

.env.example

@@ -12,6 +12,7 @@
 # Option 1: Use native APIs (recommended for direct access)
 # Get your Gemini API key from: https://makersuite.google.com/app/apikey
 GEMINI_API_KEY=your_gemini_api_key_here
+# GEMINI_BASE_URL=                            # Optional: Custom Gemini endpoint (defaults to Google's API)
 
 # Get your OpenAI API key from: https://platform.openai.com/api-keys
 OPENAI_API_KEY=your_openai_api_key_here