Gateway Aggregation Generator

[GenerateGatewayAggregation] creates a typed GatewayAggregation<TRequest, TResponse> factory from downstream fetch methods and one composer.

[GenerateGatewayAggregation(typeof(CustomerDashboardRequest), typeof(CustomerDashboardResponse), GatewayName = "customer-dashboard")]
public static partial class CustomerDashboardGateway
{
    [GatewayAggregationFetch("profile")]
    private static CustomerProfile Profile(CustomerDashboardRequest request) => new(request.CustomerId, "Ada");

    [GatewayAggregationComposer]
    private static CustomerDashboardResponse Compose(GatewayAggregationContext<CustomerDashboardRequest> ctx)
        => new(ctx.Require<CustomerProfile>("profile").CustomerId, 0);
}

Diagnostics:

• PKGA001: host type must be partial.
• PKGA002: at least one fetch and exactly one composer are required.
• PKGA003: fetch or composer signature is invalid.
• PKGA004: fetch names must be unique.