General refactoring

[enovka]

This pull request addresses the issue of code duplication and potential inconsistencies in the Gemini4j builder classes. It implements a comprehensive refactoring strategy to improve the library's maintainability, extensibility, and type safety.

Key Changes:

• Hierarchical Abstraction: Introduced a hierarchy of abstract builder classes (AbstractBuilder, AbstractComplexBuilder, AbstractRequestBuilder, etc.) to encapsulate common functionality and reduce redundancy across concrete builders.
• Generics: Utilized generics to create flexible builder classes capable of handling different request object types, enhancing code reusability.
• Decoupled Builder Chains: Decoupled builder chains by removing the generic and() method from AbstractComplexBuilder and adding specific and() methods to concrete builders, ensuring type safety and clarity.
• Refactored Resource Classes: Refactored AbstractResource and its implementations to maximize code reuse and minimize duplication. Introduced a generic type parameter for response handling and moved common request execution logic to the abstract class.
• Corrected Model Name Format: Fixed an issue where the model field in EmbedContentRequest was not in the expected format, ensuring compatibility with the Gemini API.

Benefits:

• Reduced Code Duplication: Eliminated redundant code, making the library more concise and easier to maintain.
• Improved Type Safety: Enhanced type safety by using generics and specific and() methods, reducing the risk of runtime errors.
• Enhanced Extensibility: Simplified the process of adding new builder classes for future API features.
• Increased Consistency: Ensured consistent behavior and method naming across all builders.

This refactoring significantly improves the architecture of the Gemini4j library, making it more robust, maintainable, and scalable for future development.