@angular/fire/ai fails to build against firebase 12.18, which removed getImagenModel #3749

Description

@angular/fire/ai imports getImagenModel from firebase/ai and re-exports it zone-wrapped. That import no longer resolves on a fresh install:

• Firebase shut the Imagen models down in August 2026, and @firebase/ai 2.15.0 removed the API.
• firebase 12.18.0, the current npm latest, ships @firebase/ai 2.15.0.
• @angular/fire declares firebase: "^12.4.0" as a regular dependency, so every fresh install resolves 12.18.0, and the shipped bundle imports an export that does not exist.

Impact

• A fresh app that installs @angular/fire and builds anything touching the AI entry point fails at build time with TS2305: Module '"firebase/ai"' has no exported member 'getImagenModel', or the equivalent bundler resolution error.
• Nothing the user wrote is at fault, and pinning an older firebase only trades the build error for runtime calls to a shut-down service.

Versions

Verified against @angular/fire main at fb6796b with firebase 12.18.0. The export exists through firebase 12.17.x and is gone at 12.18.0.

Expected behavior

The published entry points build against every firebase version the declared ^12.4.0 range resolves.