feat(genai): Add samples for image generation

[cfloress]

Description

Fixes #

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• genai/image_generation/imggen_mmflash_txt_img_with_txt.go:18, tag googlegenaisdk_imggen_mmflash_txt_and_img_with_txt
• genai/image_generation/imggen_mmflash_with_txt.go:18, tag googlegenaisdk_imggen_mmflash_with_txt

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[gemini-code-assist]

Summary of Changes

Hello @cfloress, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the golang-samples repository by adding new examples for image generation using the Google GenAI SDK. These samples showcase how to leverage the gemini-2.5-flash-image-preview model to create images based on text prompts, including scenarios where both text and images are generated and combined into a single output, such as an illustrated recipe.

Highlights

• New Image Generation Samples: Introduces two new Go samples demonstrating image generation capabilities using the GenAI SDK.
• Multimodal Flash with Text and Image: Adds a sample (imggen_mmflash_with_txt.go) that generates an image (e.g., Eiffel Tower) along with descriptive text using the gemini-2.5-flash-image-preview model.
• Illustrated Recipe Generation: Includes a sample (imggen_mmflash_txt_img_with_txt.go) that generates an illustrated recipe, combining text and images into a markdown file.
• New Test Coverage: Adds a new test file (image_generation_examples_test.go) to validate the functionality of the newly added image generation samples.

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[gemini-code-assist]

Code Review

This pull request adds two new Go samples for image generation with the GenAI SDK, along with corresponding system tests. The samples demonstrate generating text and images together. My review focuses on improving the robustness of the samples by ensuring necessary directories are created before writing files, which prevents potential runtime errors. I've also suggested improving test hygiene by cleaning up test artifacts.

[Guiners]

LGTM