Closes #31243

As described in my issue, the [thinking
budget](https://ai.google.dev/gemini-api/docs/thinking) gets
automatically chosen by Gemini unless it is specifically set to
something. In order to have fast responses (inline assistant) I prefer
to set it to 0.

Release Notes:

- ai: Added `thinking` mode for custom Google models with configurable
token budget

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>