Add IPolymorphicJsonConvertor interface and JsonSourceGenerationMode.Metadata support

[trejjam]

Summary

• Introduce IPolymorphicJsonConvertor<T> interface with static abstract members (NET7+) enabling compile-time polymorphic dispatch via CRTP pattern (PolymorphicJsonConvertor<TConverter, T>)
• Generate ConfigureJsonTypeInfo on each polymorphic converter to inject discriminator properties into JsonTypeInfo, enabling JsonSourceGenerationMode.Metadata serialization
• Generate GetPolymorphicJsonTypeInfoConfigurations() / GetEnumJsonTypeInfoConfigurations() on serializer contexts, giving consumers a clean API to register type info modifiers
• Update PolymorphicJsonConvertor.Write to detect when discriminator is already present in type info properties (metadata mode) and delegate directly to JsonSerializer.Serialize instead of manual property writing
• Add netstandard2.0 target to the Target project with PolySharp and System.Text.Json dependencies
• Rename instance methods to Self_ prefix as NET<7 fallback bridge to the new static abstract interface
• Add new serialization tests (SerializeLeafContractWorks, SerializeAnotherLeafContractWorks, SerializeLeafContractWithCustomDelimiterWorks) and a JsonSourceGenerationMode.Metadata snapshot test
• Regenerate all snapshot tests to reflect the updated generated code shape