Add support for Guid enum serialization

[RichardReedge]

Fixes #561

[Copilot]

Pull Request Overview

This PR adds support for Guid enum serialization across multiple serialization libraries by introducing a new TestEnumGuid class and updating all relevant formatters and converters to handle Guid types properly.

• Adds TestEnumGuid and FlagTestEnumGuid classes with Guid values across all test projects
• Updates serialization formatters/converters to handle Guid serialization as strings
• Removes IConvertible constraint from SystemTextJson converters to support Guid types
• Adds comprehensive test coverage for Guid enum serialization in all supported libraries

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
TestEnums.cs files	Add TestEnumGuid class definitions with standard Guid value
FlagTestEnums.cs files	Add FlagTestEnumGuid class for flag enum testing
Test files	Update test classes, instances, and assertions to include Guid enum testing
SmartEnumValueFormatter.cs	Add Guid serialization support for Utf8Json library
SmartFlagEnumValueFormatter.cs	Add Guid serialization support for Utf8Json flag enums
SmartEnumValueConverter.cs	Add Guid support and remove IConvertible constraint for SystemTextJson
SmartEnumNameConverter.cs	Remove IConvertible constraint to support Guid types
JsonNet converters	Add Guid parsing support for JsonNet library