Add Converter Type Support (IDynamoConverter)

[j-d-ha]

Priority: P2 - Important
Tier: 3 - Advanced Features
Effort: Large (10-16 hours)

Already In Codebase

• No IDynamoConverter<T> exists in src/LayeredCraft.DynamoMapper.Runtime/.
• DynamoFieldAttribute does not expose a converter-type property today.
• Generator has no converter parsing/validation logic.
• Docs reference IDynamoConverter<T> heavily (e.g., docs/usage/converters.md).

Goal

Support declaring a reusable converter type for a mapped member via class-level [DynamoField].

Example desired usage:

[DynamoMapper]
[DynamoField(nameof(MyEntity.Status), Converter = typeof(OrderStatusConverter))]
public static partial class MyEntityMapper { }

Runtime API (part of this story)

1. Add IDynamoConverter<T>:

public interface IDynamoConverter<T>
{
    AttributeValue ToAttributeValue(T value);
    T FromAttributeValue(AttributeValue value);
}

2. Extend DynamoFieldAttribute with:

• Type? Converter

Generator behavior

• Validate converter type implements IDynamoConverter<TProperty>.
• Emit code that uses the converter:
  • Prefer a cached static readonly converter instance per mapper to avoid allocations.
  • Call converter.ToAttributeValue(value) / converter.FromAttributeValue(attributeValue).

Conflicts

• Converter cannot be used together with ToMethod / FromMethod (emit diagnostic).

Diagnostics

• Converter type does not implement IDynamoConverter<T>.
• Converter generic type mismatch with property type.
• Conflicting converter config.

Tests (must be part of this story)

Add verify tests covering:

• Happy path: converter type used in generated code.
• Failure: wrong interface or mismatch emits diagnostic.
• Failure: converter + ToMethod/FromMethod emits diagnostic.

Acceptance criteria

• Converter types work for custom member types.
• Diagnostics are clear and point to the attribute usage.
• Snapshot tests cover happy path and key failures.