Add automatic fallback for Gemini 3.0 models when unavailable

- Add automatic fallback logic in chatThreadService to detect 404 errors for Gemini 3.0 models and retry with gemini-2.5-pro
- Reorder Gemini models list to prefer gemini-2.5-pro over gemini-3.0-pro until 3.0 is confirmed available in all regions
- Prevents user-facing errors when Gemini 3.0 models are not yet available in the API
- Fallback happens transparently without user intervention

Author: Tajudeen