feat: Remove custom JSON converters for enums

[stephentoub]

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[gemini-code-assist]

Summary of Changes

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This pull request aims to streamline and standardize how enums are serialized to JSON. By replacing custom, enum-specific converters with a single, reusable generic converter that handles kebab-case lower serialization, I've reduced code duplication and improved the maintainability of the serialization logic across the application.

Highlights

• Serialization Refactor: I've refactored the JSON serialization for enums by replacing custom, hand-written JsonConverter implementations with a standardized, generic approach.
• Code Reduction: This change eliminates boilerplate code by removing specific JsonConverter classes, such as MessageRoleConverter, which were previously responsible for handling enum serialization.
• New Utility Converter: I've introduced a new generic JsonConverter, KebabCaseLowerJsonStringEnumConverter<TEnum>, which leverages System.Text.Json's built-in capabilities to serialize enum values into kebab-case lower strings. This converter is now applied to MessageRole and TaskState enums.

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[gemini-code-assist]

Code Review

This pull request refactors the JSON serialization of enums by replacing custom converters with a new generic KebabCaseLowerJsonStringEnumConverter. My review has identified a potential critical breaking change in the JSON format for enums with multi-word names (e.g., TaskState.InputRequired), and a high severity compatibility issue with using C# 12 primary constructors in the new converter, as the library targets netstandard2.0, which does not officially support this feature.