feat(gemini): add ImageConfig support

[dcsunny]

Summary

This PR adds comprehensive ImageConfig support to the Gemini model component, allowing users to configure image generation parameters.

Changes

• ✅ Add ImageConfig field to Config struct for default configuration
• ✅ Add imageConfig field to ChatModel struct for runtime storage
• ✅ Initialize imageConfig in NewChatModel from Config
• ✅ Add WithImageConfig option function in option.go
• ✅ Update genInputAndConf to support both default and per-call ImageConfig

Implementation Details

This implementation follows the same pattern as ThinkingConfig, supporting both:

1. Default configuration during model creation via Config.ImageConfig
2. Dynamic override during Generate/Stream calls via WithImageConfig()

Usage Example

// Set default ImageConfig during model creation
model, _ := gemini.NewChatModel(ctx, &gemini.Config{
    Client: client,
    Model:  "gemini-2.0-flash-exp",
    ImageConfig: &genai.ImageConfig{
        // Image generation configuration
    },
})

// Override ImageConfig per call
model.Generate(ctx, messages, gemini.WithImageConfig(&genai.ImageConfig{
    // Custom configuration for this call
}))

Testing

- ✅ Code compiles successfully
- ✅ Implementation is consistent with existing patterns (ThinkingConfig, ResponseModalities)