* feat: supports thinking level of google gemini
- Updated google-genai version to >=1.56.0 in pyproject.toml and requirements.txt.
- Changed model configuration from "gemini-1.5-flash" to "gemini-3-flash-preview" in default.py.
- Enhanced thinking configuration handling in gemini_source.py to support new parameters for Gemini 3 models.
* fix: standardize thinking level configuration in default.py and gemini_source.py
- Updated the thinking level values in default.py to uppercase for consistency.
- Enhanced gemini_source.py to validate the thinking level and default to "HIGH" if an invalid value is provided.