Add support for Google GenAI Image Generation (Gemini) in Spring AI

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Add support for Google GenAI Image Generation (Gemini) in Spring AI

Expected Behavior

Spring AI should provide first-class support for Google GenAI (Gemini) image generation models (gemini-2.5-flash-image, gemini-3-pro-image-preview, etc.) as a model type, similar to existing support for text and embeddings. Ideally support should include:

• A new ImageModel implementation for Gemini image generation
• A Spring Boot starter and auto-configuration
• Mapping Gemini image responses into ImageResponse
• A prompt abstraction supporting:
  • text instructions
  • inline base64 images
  • aspect ratio / resolution parameters
• Configuration properties for defaults

Example (illustrative):

GoogleGenAiImagePrompt prompt = GoogleGenAiImagePrompt.builder()
    .text("Add happy pug to the picture of the man.")
    .addImage(existingBase64Image, "image/jpeg")
    .build();

ImageResponse images = imageModel.call(prompt);

Current Behavior

Spring AI currently supports Google GenAI for:

• Chat (text)
• Embeddings

but not for image generation. Developers must call the Google GenAI Java SDK directly, bypassing Spring AI abstractions. There is no unified integration with:

• ImageModel
• ImagePrompt
• ImageResponse
• Spring Boot configuration properties

Context

I am building a Spring Boot application that uses Gemini image generation. While the GenAI Java SDK works directly, an official Spring AI integration would:

• improve DX
• align with existing ImageModel implementations
• support multimodal Gemini use cases
• leverage Spring Boot starters and configuration

There are no reasonable workarounds that reuse Spring AI abstractions without reimplementing core pieces of the framework.

I am willing to contribute a PR. I already have a working implementation of:

• GoogleGenAiImageModel
• GoogleGenAiImageApi (low-level SDK adapter)
• GoogleGenAiImageOptions
• Spring Boot auto-configuration
• unit tests

Start by reviewing Spring AI's existing ImageModel, ImagePrompt, and ImageResponse integrations, along with the current Google GenAI chat and embedding support. Compare the proposed GoogleGenAiImageModel, GoogleGenAiImageApi, GoogleGenAiImageOptions, auto-configuration, and unit tests with those patterns. Done means Gemini image generation works through the Spring AI abstractions and configuration described in the issue.