fix(gemini): preserve `additionalProperties:false` for Antigravity response schemas

- Add a sanitizer option to keep `additionalProperties: false` in response paths instead of converting it to description hints.
- Skip dropping `additionalProperties: false` in `removeUnsupportedKeywords` when response mode is enabled, while still removing non-false `additionalProperties`.
- Keep tool-schema behavior unchanged by continuing to remove `additionalProperties` and emit compatibility hints for non-response conversions.

Closes: #4961