google_ai: Add Gemini 3.6 Flash (#62010)

Closes #62007

Adds Gemini 3.6 Flash to the Google AI provider so it can be selected in
the agent, alongside the existing Gemini 3.5 Flash.

Google released Gemini 3.6 Flash on July 21, 2026. It keeps the 1
million token context window, has a 64k output limit, and supports
thinking. The model is added with the same capabilities and thinking
levels as 3.5 Flash (Minimal, Low, Medium, High, defaulting to Medium).

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. The thinking behavior in `completion.rs` keys off the
`gemini-3` id prefix, so it already applies to this model.

Release Notes:

- Added Gemini 3.6 Flash to the Google AI models

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>