Add DateOnly and TimeOnly support

[Copilot]

Extends the library to support DateOnly, DateOnly?, TimeOnly, and TimeOnly? types with custom format serialization.

Changes

• Converters: Added 4 converters following existing DateTime/DateTimeOffset patterns

  • DateOnlyConverter, DateOnlyNullableConverter
  • TimeOnlyConverter, TimeOnlyNullableConverter
  • Non-nullable types throw JsonException on invalid input; nullable types return null

• Factory registration: Updated DateTimeConverterFactoryHelper and JsonDateTimeFormatConverter.CanConvert() to recognize new types

• Tests: Added 18 tests covering all serialization approaches (reflection, source generator, resolver) with distinctive formats to verify custom converters work correctly

• Documentation: Updated README.md and AGENT.md with new type support and examples

Usage

All existing usage patterns now support DateOnly/TimeOnly:

// Reflection-based
public class Order
{
    [JsonDateTimeConverter("MM/dd/yyyy")]
    public DateOnly DeliveryDate { get; set; }
    
    [JsonDateTimeConverter("HH:mm")]
    public TimeOnly DeliveryTime { get; set; }
}

// Source generator with format converter
public class DateOnlyFormat : IJsonDateTimeFormat
{
    public static string Format => "MM/dd/yyyy";
}

[JsonConverter(typeof(JsonDateTimeFormatConverter<DateOnlyFormat>))]
public DateOnly DeliveryDate { get; set; }

// Source generator with resolver (.NET 9+)
[JsonDateTimeFormat("MM/dd/yyyy")]
public DateOnly DeliveryDate { get; set; }

Original prompt

Add support for
DateOnly
DateOnly?
TimeOnly
TimeOnly?

Update/Add tests, Readme.MD, and Agent.MD to reflect the changes

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