Feature/google genai 3.1 flash lite and customtools

[azanux]

Add Gemini 3.1 Flash Lite Preview and Gemini 3.1 Pro Preview Customtools models

Summary

Add support for two new Google Gemini 3.1 models:

• gemini-3.1-flash-lite-preview - Lightweight and cost-effective latest generation model ($0.25/$1.50 per 1M tokens input/output)
• gemini-3.1-pro-preview-customtools - Behavioral variant of Gemini 3.1 Pro optimized for prioritizing custom tools ($2.00/$12.00 per 1M tokens input/output)

Changes

• Added model constants in GoogleGenAiModels.kt (Kotlin) and GeminiModels.java (Java)
• Added model definitions in both google-genai-models.yml (Google GenAI / AI Studio) and gemini-models.yml (Vertex AI)
• Added unit tests for both models in GoogleGenAiModelLoaderTest and GeminiModelLoaderTest
• Updated getting-started documentation with new model entries

Test plan

• GoogleGenAiModelLoaderTest — 32 tests passing
• GeminiModelLoaderTest — 15 tests passing