Google has announced shutdown dates for the Gemini 2.5 models and Gemini
3.1 Flash-Lite, but they remained available in the Google AI model
picker. Deprecated aliases could also still be deserialized as supported
models.

Remove those deprecated models and aliases from the built-in model list.
Gemini 3.5 Flash-Lite is now the default and fast model, matching
Google's recommended replacement for Gemini 3.1 Flash-Lite.

Release Notes:

- agent: Remove deprecated Gemini models