feat(models): add Gemini 3.5 Flash Lite and 3.1 Flash Lite to Gemini API provider (#1334)

* feat(models): add Gemini 3.5 Flash Lite and 3.1 Flash Lite to Gemini API provider

Adds the two new stable Flash Lite models so users can select them directly instead of relying on gemini-flash-lite-latest (#1251).

Model ids, limits and pricing verified against the official Gemini API docs and the OpenRouter catalog.

* fix(models): support minimal thinking level for Gemini 3.1 Flash Lite

The official model docs list minimal, low, medium, and high thinking levels for gemini-3.1-flash-lite; add the missing minimal level (CodeRabbit review on #1334).

* fix(models): default Flash Lite reasoning effort to minimal

The documented API defaults are On (minimal) for gemini-3.5-flash-lite and minimal is the lowest supported level for gemini-3.1-flash-lite; defaulting to medium would raise unset users above the cheap tier. Addressed via CodeRabbit review on #1334.

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Co-authored-by: Eason Liang <easonliang28@gmail.com>