Refactor: Improve maintainability and resolve code smells across core and provider modules

[AdnanIITDU]

Summary

This Pull Request improves the maintainability, readability, and overall code quality of the dilithium-java project through a series of behavior-preserving refactorings. The changes focus on reducing technical debt, improving code structure, and modernizing parts of the implementation without altering the cryptographic algorithms or public API behavior.

Changes Made

Refactoring

• Extracted complex logic from Dilithium.sign() into dedicated helper methods to reduce cognitive complexity.
• Reduced duplicated bit-packing logic in PackingUtils by introducing reusable helper methods.
• Extracted duplicated nonce encoding logic into a dedicated helper method.
• Replaced magic numbers with named constants for improved readability and maintainability.
• Improved exception handling by standardizing invalid input validation.
• Replaced manual array initialization with standard Java library methods where appropriate.
• Removed unused imports and redundant code.
• Fixed formatting and indentation inconsistencies.
• Improved field immutability by marking appropriate fields as final.
• Removed deprecated AccessController usage from DilithiumProvider for better compatibility with modern Java versions.

Code Quality Improvements

• Reduced code duplication (DRY principle)
• Improved readability and maintainability
• Reduced method complexity
• Improved encapsulation and code organization
• Modernized Java API usage
• Improved consistency in exception handling
• Removed unnecessary technical debt

Functional Validation

The refactoring was performed with the goal of preserving all existing functionality.

• No cryptographic algorithms were modified.
• No mathematical computations were changed.
• Public APIs remain unchanged.
• Refactoring was limited to structural and maintainability improvements.
• Existing functionality and behavior remain intact.

Benefits

• Easier to understand and maintain codebase
• Lower cognitive complexity in critical methods
• Better adherence to Clean Code and SOLID principles
• Reduced duplication and improved code reuse
• Improved compatibility with newer Java versions
• Better foundation for future enhancements

Notes

This PR focuses solely on refactoring and maintainability improvements. It intentionally avoids changing any cryptographic behavior or introducing new features to ensure correctness and backward compatibility.

Thank you for your time and review. Any feedback or suggestions are greatly appreciated.