Making realtime-conversational-agent work with Gemini API (Google AI studio)

[jgonzalezd]

Summary

This PR improves compatibility between Google AI Studio (Gemini API) and Vertex AI by making configuration parameters conditional based on the API provider being used.

Changes

1. Added missing APP_NAME environment variable to .env template

• Added APP_NAME to both Google AI Studio and Vertex AI .env template examples in README
• This variable is required for session management but was missing from the boilerplate

2. Made SessionResumptionConfig.transparent conditional on API provider

• The transparent parameter is supported by Vertex AI but not by the Gemini API
• Added conditional logic in server/main.py to enable transparent=True only when using Vertex AI
• Prevents errors when using Google AI Studio while preserving functionality for Vertex AI users

3. Made model name selection conditional on API provider

• Model naming conventions differ between providers:
  • Vertex AI: gemini-live-2.5-flash-preview-native-audio (with "live" prefix)
  • Gemini API: gemini-2.5-flash-native-audio-preview-12-2025 (with date suffix)
• Extracted model selection logic into _get_model_name() helper function with proper type hints
• Improved code organization with constants and documentation
• Automatically selects the correct model based on GOOGLE_GENAI_USE_VERTEXAI environment variable

Benefits

• ✅ Works seamlessly with both Google AI Studio and Vertex AI without code changes
• ✅ Eliminates configuration errors when switching between providers
• ✅ Better code organization and maintainability
• ✅ Proper documentation of provider-specific differences

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