Fix text and image multimodal support so that it properly works regardless of capability chosen

[felixarntz]

So far, the Gemini multimodal text and image models were only working when used for the image_generation capability, even though they technically support both text_generation and image_generation.

They always were able to create both in our implementation, but the routing (model metadata plus class interfaces implemented) was not quite right.

This PR fixes the last important bit, by introducing a class that implements both the generateTextResult and generateImageResult methods.

cc @dkotter

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Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: JasonTheAdams <jason_the_adams@git.wordpress.org>

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