Firebase AI Logic - Gemini 3.5 Flash Model Support

Firebase AI Logic supports the latest Gemini stable model: gemini-3.5-flash. This model can be accessed via either the Gemini Developer API or the Vertex AI Gemini API. Migration from preview version requires updating model name from gemini-3-flash-preview to gemini-3.5-flash, reviewing pricing changes, testing prompts (default thinking level changed from HIGH to MEDIUM), and removing temperature, topP, topK parameters.