Support `minimal` and `medium` thinking levels for `gemini-3-flash` familiy

[marcjaner]

The currently supported thinking levels for Spring AI Google GenAI in v1.1.2 are:

• LOW
• HIGH

While Google officially also supports:

• MINIMAL
• MEDIUM

for the Gemini 3 Flash familiy.

[ddobrin]

@marcjaner
This functionality was part of a PR I submitted, which did not make it in time for 2.0.0-M1.

These additional levels have been introduced as part of the Gemini 3 Flash Preview model, and were not available at the time Gemini 3 Pro Preview has been released.

Assign this to me, please
cc @markpollack @ilayaperumalg

[RyanHowell30]

@ddobrin thanks for looking into this ! When is this expected to roll out?

Some really cool, frsutrating and interesting behaviour I have found in the Gemini 3 Flash model family. If a prompt has roughly over 10,000 tokens and is very complex in nature to reason about and the chat model is set to LOW thinking level, it will just not think at all and return zero thinking tokens.

[ddobrin]

@RyanHowell30 - I think by the beginning of next week, hopefully it makes it into the new milestone.

I have to test that functionality out as it would match the minimal thinking level. I would not be surprised however needs to be tested.