feat: Update Gemini model list to include latest models (Gemini 3, 2.0)

[tangyu]

Problem

The hardcoded Gemini model list in the codebase is outdated. Currently, only 3 models are available for selection:

• gemini-2.5-pro
• gemini-2.5-flash
• gemini-2.5-flash-lite

However, Google has released newer models including Gemini 3 series and Gemini 2.0 series.

Affected Files

File	Line	Current Content
src/index.ts	117	validModels = ['gemini-2.5-pro', 'gemini-2.5-flash', 'gemini-2.5-flash-lite']
src/agent/transport/handlers/GeminiTransport.ts	81-83	Same list
src/gemini/runGemini.ts	1168	Error message with outdated model list
README.md	75	Documentation with outdated list

Suggested Models to Add

Based on @ai-sdk/gateway type definitions:

# Gemini 3 series (latest)
gemini-3-pro-preview
gemini-3-pro-image

# Gemini 2.0 series
gemini-2.0-flash
gemini-2.0-flash-lite

Suggestion

Consider centralizing the model list in src/gemini/constants.ts to avoid duplication and make future updates easier.