Follow-up to #62010, which added Gemini 3.6 Flash.

Google released Gemini 3.7 Flash on August 13, 2026. It keeps the 1
million token context window and the 64k output limit, and supports
thinking.

Unlike 3.5 and 3.6 Flash, it does not accept `thinking_level: MINIMAL`
(the API returns a validation error). So it exposes Low, Medium and
High, defaulting to Medium, and `disabled_thinking_level` in
`completion.rs` returns Low for it instead of Minimal when the user
turns thinking off.

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 3.7 Flash to the Google AI models