[FEAT] Add Gemini 2.5 Pro / Flash Preview via Google AI API

Model name on Google AI API:

• gemini-2.5-flash-preview-05-20
• gemini-2.5-pro-preview-05-06

Reference:

• https://blog.google/technology/google-deepmind/gemini-model-thinking-updates-march-2025/#building-on-best-gemini
• https://ai.google.dev/gemini-api/docs/models#gemini-2.5-flash-preview
• https://ai.google.dev/gemini-api/docs/models#gemini-2.5-pro-preview-05-06