.messages.message reaction field type vs. error from macro

The documented template for @AppEntity(schema: .messages.message) at Integrating your messaging app with Apple Intelligence and on the MessagesEntity.message reference page shows:

var reaction: <#ReadReaction#>?

with <#ReadReaction#> rendered as a placeholder (the Xcode placeholder syntax). A natural reading is "fill in your own type that you want to use here". I tried several:

• Tapback? where Tapback is @AppEnum(schema: .messages.customReaction)
• MessageReaction? (same, renamed)
• MessageReaction? with manual AppEnum conformance (no schema decoration)
• AttributedString?
• AttributedString (non-optional)
• field omitted entirely

Each variant produced one of:

error: Property 'reaction' type does not match required AppSchemaEntity property type 'ReadReactionCases:(Schema<Tapback> | AttributedString)'
error: Required AppSchemaEntity property 'reaction' must be optional
error: Missing required property 'reaction' from AppSchemaEntity 'messages.message'

The phrase ReadReactionCases:(Schema<Tapback> | AttributedString) is opaque — ReadReactionCases isn't a public type, Schema<Tapback> isn't constructable from outside, and the documentation doesn't mention @UnionValue in this context.

The actual working pattern, which I only found by downloading the UnicornChat sample and reading MessageEntity.swift, is:

@UnionValue
enum MessageReaction: Sendable {
    case customReaction(CustomReaction)
}

@AppEnum(schema: .messages.customReaction)
enum CustomReaction: String, AppEnum {
    case sticker
    static let caseDisplayRepresentations: [Self: DisplayRepresentation] = [
        .sticker: "Sticker"
    ]
}

Two suggestions:

1. Update the schema-template comment in the docs to either name MessageReaction explicitly with a @UnionValue annotation, or include a one-line note: "The reaction field is a @UnionValue enum wrapping the customReaction schema enum — see UnicornChat sample." The current placeholder <#ReadReaction#> plus the prose suggesting "you can generate the properties... with the @AppEntity(.messages.message) Swift macro" reads as if the developer just supplies any AppEnum-conformer.

2. Improve the diagnostic. Property type does not match required AppSchemaEntity property type 'ReadReactionCases:(Schema<Tapback> | AttributedString)' is unhelpful because ReadReactionCases doesn't appear in any reachable type. Either rename to something developer-facing, or include a fix-it that suggests @UnionValue.

iOS 27.0 beta, Xcode 27 beta, macOS Tahoe 26.4.