Nano Banana Pro 🍌 update

[lethargicpanda]

Update Gemini image sample with Nano Banana Pro 🍌

[gemini-code-assist]

Summary of Changes

Hello @lethargicpanda, 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 updates the Gemini image sample application to leverage a newer, more advanced generative AI model. The change specifically targets the GeminiImageChatViewModel to utilize the gemini-3-pro-image-preview model, enhancing the sample's capabilities and aligning it with the latest available technology, playfully referred to as "Nano Banana Pro."

Highlights

• Gemini Model Update: The GeminiImageChatViewModel has been updated to utilize the gemini-3-pro-image-preview model, replacing the older gemini-2.5-flash-image-preview model.

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[lethargicpanda]

cc @JolandaVerhoef @ksemenova

[gemini-code-assist]

Code Review

This pull request updates the generative model in the Gemini image chat sample from gemini-2.5-flash-image-preview to gemini-3-pro-image-preview. While the code change is correct, the corresponding README.md file for the sample has not been updated to reflect this change. This is required by the repository's style guide. I have added a comment highlighting this issue.

[JolandaVerhoef]

Please update the readme, approving to unblock you from merging after that's done :)

[ksemenova]

Suggestion: let's move the model name+version to a higher level constant and remove version numbers from readme.