Implement google adapter with new openai compatible feature

[wilsonccccc]

Summary

Google recently announced that Gemini models are now accessible via an OpenAI-compatible REST API endpoint. This PR refactors our Google adapter to leverage this standardized interface, significantly simplifying the codebase by removing hundreds of lines of custom payload conversion and streaming logic.

Changes

Added

• GeminiOpenAIAdapter - New adapter that subclasses OpenAIAdapter and automatically handles the /openai endpoint suffix
• Auto-normalization of base URLs to ensure compatibility with Google's OpenAI endpoint

Modified

• adapter_factory.py - Updated to use GeminiOpenAIAdapter for the "gemini" provider instead of the legacy GoogleAdapter
• google_adapter.py - Added deprecation notice indicating this is legacy code

Removed

• Unused GoogleAdapter import from adapter factory