Refactor: Modernize Internal Codebase Using Dart 3.10 Features (No Breaking Changes)

[Aniketkhote]

PR Description

This PR updates the internal implementation to use modern Dart 3.10 features while keeping the public API and behavior fully backward-compatible.
The goal is to simplify maintenance, reduce boilerplate, and improve performance without introducing new APIs or breaking existing ones.

Key Improvements

• Refactored internal logic to leverage Dart 3.10 enhancements (records, patterns, sealed classes, etc. as applicable)
• Reduced redundant code and improved structure for long-term maintainability
• Ensured no changes to external API, request/response structure, or public-facing behavior
• Added minor internal optimizations and cleaned up outdated patterns

Notes

• No user-facing changes
• All existing tests pass
• This PR is safe for incremental adoption