fix: correctly parse eveara errorfield

[AndrewWestberg]

No description provided.

[Copilot]

Pull request overview

This PR updates the Eveara distribution models so that error_fields.message can be parsed correctly whether Eveara sends it as a simple string or as a nested object, and adds tests to lock in the expected behavior across the relevant response types.

Changes:

• Changed ErrorField.message from a nullable String to a nullable ErrorFieldMessage sealed type, and added a custom ErrorFieldMessageSerializer to handle both string and object JSON representations.
• Introduced ErrorFieldMessage.SimpleMessage and ErrorFieldMessage.ComplexMessage to model the two observed Eveara formats, including nested messages, success flag, and errorFields key.
• Added ErrorFieldTest to verify parsing of standalone ErrorField, ValidateData, and ValidateAlbumResponse for both simple and complex error_fields payloads, including mixed lists and null/missing messages.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
newm-server/src/main/kotlin/io/newm/server/features/distribution/model/ErrorField.kt	Extends ErrorField to use a sealed ErrorFieldMessage with a custom JSON serializer to support both string and complex-object message formats from Eveara.
newm-server/src/test/kotlin/io/newm/server/features/distribution/model/ErrorFieldTest.kt	Adds unit tests covering decoding of simple, complex, mixed, and null error_fields in ErrorField, ValidateData, and ValidateAlbumResponse.