Improve Required Field Errors (DynamoMappingException)

[j-d-ha]

Priority: P2 - Important
Tier: 3 - Advanced Features
Effort: Medium (6-8 hours)

Already In Codebase

• Requiredness is already calculated and passed into runtime Get methods:
  • Field override + global default requiredness: src/LayeredCraft.DynamoMapper.Generators/PropertyMapping/PropertyMappingSpecBuilder.cs
  • Runtime behavior on missing keys:
    • Requiredness.Required and Requiredness.InferFromNullability currently throw InvalidOperationException.
    • Requiredness.Optional returns a DynamoDB NULL value.
    • src/LayeredCraft.DynamoMapper.Runtime/AttributeValueExtensions/UtilAttributeValueExtensions.cs
• There are tests that exercise requiredness override scenarios:
  • test/LayeredCraft.DynamoMapper.Generators.Tests/DynamoFieldVerifyTests.cs

Goal

Improve required field validation behavior in FromItem:

• Ensure requiredness rules are consistently enforced.
• Throw a purpose-built DynamoMappingException (already exists) with useful context (mapper/type/field/member).

Implementation notes

• Prefer updating runtime Get helpers to throw DynamoMappingException instead of InvalidOperationException.
• Include:
  • mapper name
  • target type
  • field name
  • member name
  • details about what was missing
• Ensure behavior is consistent for:
  • required non-nullable members
  • [DynamoField(Required = true/false)] overrides
  • DefaultRequiredness mapper option

Tests

• Add verify/behavior tests for missing required fields that validate the thrown exception type/message.

Acceptance criteria

• Missing required fields throw DynamoMappingException with actionable details.
• Optional fields remain optional and do not throw.
• Tests cover both required and optional cases.