Refactor, Ensure Gemini Latest model is correctly set, Fix custom model name usage and update model constants

[AkhmadRamadani]

This PR addresses critical issues #53 and #54, where setting the model to gemini-2.0-flash (or other custom model names) did not actually apply.

Changes

• Refactored GeminiImpl to streamline request handling and model management.
• Fixed logic to ensure the latest Gemini model (e.g., gemini-2.0-flash) is correctly set.
• Corrected handling of custom model names to prevent silent fallbacks.
• Updated Gemini model constants and descriptions for the latest stable version.
• Added request data builders for improved consistency and maintainability.

Impact

• Fixes urgent issues preventing users from switching to Gemini 2.0 Flash or using custom model names.
• Improves internal structure for easier future updates and better reliability.

[AkhmadRamadani]

Hi @babakcode, I want to ask, when will the package versions be updated on pub.dev? Thank you