Support all Gemini native image models (including 🍌)

[tombeckenham]

Problem

The Gemini image adapter currently only routes gemini-3.1-flash-image-preview (🍌 Nano Banana 2) through the generateContent API path. The other three Gemini native image models should also use this path:

• gemini-3-pro-image-preview
• gemini-2.5-flash-image
• gemini-2.0-flash-preview-image-generation

Additionally, the current implementation has several bugs:

1. Wrong config property: Uses imageGenerationConfig but the SDK's GenerateContentConfig expects imageConfig
2. Wrong size property: Uses outputImageSize but the SDK's ImageConfig expects imageSize
3. Invalid type values: GeminiNanoBananaAspectRatio includes ratios not in the SDK (1:4, 1:8, 4:1, 4:5, 5:4, 8:1) and GeminiNanoBananaResolution includes 512px (SDK only supports 1K, 2K, 4K)

Expected Behavior

• All gemini-* image models should route through the generateContent API with responseModalities: ['IMAGE']
• Imagen models (imagen-3.0-*, imagen-4.0-*) should continue using the generateImages API
• Config properties should match the @google/genai SDK types (GenerateContentConfig, ImageConfig)
• Type definitions should only include values actually supported by the SDK
• Types should be renamed from GeminiNanoBanana* to GeminiNativeImage* since they apply to all Gemini native image models, not just 🍌

References

• GenerateContentConfig — has imageConfig?: ImageConfig
• ImageConfig — has aspectRatio?: string, imageSize?: string
• Requires @google/genai >= 1.43.0 for ImageConfig type