refactor(firebaseai)!: Removed deprecated Imagen methods and types due to Imagen models being shut down

[cynthiajoan]

Description

Replace this paragraph with a description of what this PR is doing. If you're modifying existing behavior, describe the existing behavior, how this PR is changing it, and what motivated the change.

Related Issues

Removed deprecated Imagen methods and types due to Imagen models being shut down in August 2026. As a replacement, you can migrate your apps to use Gemini Image models (the "Nano Banana" models).

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Breaking Change

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[russellwheatley]

This is a breaking change so I've added a ! to the PR title to ensure it is versioned as a breaking change 🙏