feat: Add comprehensive test coverage improving from 41.4% to 95.7%

[corylanou]

This PR significantly enhances the test coverage of the remark package by adding comprehensive test suites for all core components. The main package coverage improved from 41.4% to 95.7% (54.3% improvement), with overall project coverage increasing from 63.0% to 74.5%.

🧪 New Test Files Added

• attributes_test.go - Complete test coverage for Attributes type and methods
• code_test.go - Tests for Code type including HTML generation and string conversion
• generic_test.go - Comprehensive tests for Generic type and NewGeneric() function
• metadata_test.go - Full test coverage for Metadata type and interface compliance
• string_test.go - Tests for String type and all interface methods
• include_test.go - Tests for Include type with various content scenarios
• snippet_test.go - Tests for Snippet type and Snippets map operations
• heading_test.go - Tests for Heading type structure and different heading levels
• image_test.go - Tests for Image type and attribute handling

🔧 Enhanced Existing Tests

• tag_test.go - Added comprehensive tests for Tags slice methods (Overview, String, FindFirst, FindAll)
• code_block_test.go - Added missing test for String() method

📊 Coverage Improvements

• Main package: 41.4% → 95.7% ✅
• Functions now at 100% coverage: All Attributes, Code, Generic, Metadata, String, Include, Snippet, and Tags methods
• Overall project coverage: 63.0% → 74.5% ✅

🎯 Testing Best Practices Applied

• Parallel testing with t.Parallel() for improved performance
• Table-driven tests for comprehensive scenario coverage
• Subtest organization using t.Run() for better structure
• Edge case testing including nil values, empty inputs, and special characters
• Interface compliance verification ensuring types implement expected interfaces
• Comprehensive assertions using testify/require for clear failure messages

🔄 Additional Updates

• Updated Go version to 1.24 across all modules
• Enhanced README.md with comprehensive documentation
• Updated GitHub Actions workflow for modern Go versions
• Updated dependencies to latest versions

🎉 Impact

This PR makes the codebase significantly more reliable and maintainable by providing comprehensive test coverage across all core functionality. The test suite follows Go best practices and provides a solid foundation for future development.