feat(ai)!: remove Imagen API

The Imagen service has shut down, so the already-deprecated Imagen API is
removed rather than left as a surface that cannot work. This deletes the whole
vertical slice: the two models, the public and internal types, the image-format
helper, the predict-only request and response helpers, and the predict task
members. Gemini image generation via getGenerativeModel() and the non-Imagen
template request paths are untouched.

Type parity is documented rather than resolved: the repo pins firebase@12.17.1,
which resolves @firebase/ai@2.14.0 and still exports Imagen upstream. The 14
resulting differences are recorded as missingInRN in the compare-types config
and become stale once the pin reaches >= 12.18.0.

BREAKING CHANGE: The Imagen API is removed from @react-native-firebase/ai.
getImagenModel(), getTemplateImagenModel(), ImagenImageFormat, and the Imagen
request and response types are gone. Generate images with Gemini image models
through getGenerativeModel() instead, setting responseModalities and
imageConfig on the generation config. See docs/migrating-to-v27.mdx for the
full mapping of removed exports and before/after code.