Follow-up to #62670, which added Gemini 3.7 Flash.

Adds Gemini 3.5 Flash-Lite (`gemini-3.5-flash-lite`), Google's cheapest
3.5 model and the one they currently offer for free on the API free
tier.

It keeps the 1 million token context window and the 64k output limit.
Like Gemini 3.1 Flash-Lite it supports Minimal, Low, Medium and High
thinking, and defaults to Minimal. `disabled_thinking_level` in
`completion.rs` already returns Minimal for it through the generic
`gemini-3` case, so no change was needed there.

The provider builds its model list from `google_ai::Model::iter()`, so
adding the enum variant is enough for it to show up in the model
dropdown.

Release Notes:

- Added Gemini 3.5 Flash-Lite to the Google AI models