[Firebase AI] Update models to newer versions

[andrewheard]

Replaced gemini-2.0-flash-001 with gemini-2.5-flash-lite and, more importantly, replaced imagen-3.0-generate-002 with imagen-4.0-fast-generate-001. Imagen 3 is deprecated on the Gemini Developer API as of 2025-11-10.

[gemini-code-assist]

Summary of Changes

Hello @andrewheard, 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 focuses on updating the AI models used within the Firebase AI example application. The primary goal is to ensure compatibility and leverage the latest capabilities by migrating from older model versions, particularly addressing the upcoming deprecation of Imagen 3.0, and improving the Gemini model integration.

Highlights

• Gemini Model Update: The gemini-2.0-flash-001 model has been replaced with the newer gemini-2.5-flash-lite across various example screens and view models.
• Imagen Model Update and Deprecation: The imagen-3.0-generate-002 model has been upgraded to imagen-4.0-fast-generate-001. This change is crucial as Imagen 3.0 is slated for deprecation on the Gemini Developer API by November 10, 2025.
• Imagen Aspect Ratio Adjustment: The default aspect ratio for image generation in the Imagen example has been updated from landscape4x3 to square1x1 to align with the new Imagen 4.0 model.

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[gemini-code-assist]

Code Review

This pull request correctly updates the Gemini and Imagen models to newer versions, which is an important update to keep the example app current and avoid using deprecated models. The changes look good. My main feedback is to consider centralizing the model name strings. Currently, they are duplicated across multiple files, which makes updates like this one more tedious and error-prone. Defining them as constants in a single place would significantly improve the code's maintainability.

[paulb777]

Let's talk again about adding the enum ....