chore(py/plugins/google-genai): retire the Imagen catalog

[hilariie]

Closes #6176

Summary

Removes Imagen from the Python google-genai plugin. models/imagen.py is deleted along with ImagenVersion, KnownImagen, ImagenConfigSchema, ImagenModel, and the imagen_model constructors on GoogleAI and VertexAI. This is a breaking change to the package's public exports.

An imagen- id now fails closed on both plugins: ai.generate reports the model as not found instead of falling through to a Gemini text model, and every typed ref constructor rejects it with a hint pointing at gemini_image_model. Model discovery never registers imagen-* entries the API may still list.

The vertexai-imagen sample and its workspace entry are removed. The image flow in google-genai-media now targets googleai/gemini-2.5-flash-image. Removing the workspace member invalidated uv.lock, so uv lock re-emitted a few redundant environment markers in their current form; no dependency versions changed.

Also resolves the ImagenConfigSchema bullet of #6185. JS and Go still list the Imagen 4 ids and are out of scope here.

The new fail-closed and discovery tests were run against the parent commit's source and fail there.

Also, the Vertexai sample folder is still named vertexai-imagen. Renaming it would touch the root workspace config, the samples index, and possibly CI matrices, which is wider than this change, so I left it (can be a follow-up issue).

[gemini-code-assist]

Code Review

This pull request removes the dedicated imagen model implementation, configuration schemas, and exports from the genkit-google-genai package. Imagen models are now classified as unsupported, and the system redirects users to use gemini_image_model() for image generation instead. Feedback on the changes suggests refactoring the unsupported model error handling in _model_refs.py to avoid redundant string formatting when suggesting the alternative image generation method.