Tidy ModifyEntityResult usage

[donaldgray]

This is a small, non functional change that is purely around improving developer experience when working with the code.

public class ModifyEntityResult<T, TEnum> : IModifyRequest where T : JsonLdBase is used in ~80 places in the codebase.

This class is based on class of same name from Protagonist, where

• T is the type of entity being returned
• TEnum is an enum containing error code and is used to construct problem+json response.

I'm suggesting that this class be simplified, suggestion (not many fields missed for brevity):

public class ModifyEntityResult<TEnum> : IModifyRequest
{
    // Rather than T where T : JsonLdBase, just make entity a JsonLdBase. 
    // everything we edit is a JsonLdBase so just make this one, simplifies signature
    public JsonLdBase? Entity { get; private init; }

    // Code analysis attrs to avoid nullable warnings
    [MemberNotNullWhen(false, nameof(ErrorType))]
    [MemberNotNullWhen(true, nameof(Entity))]
    public bool IsSuccess { get; private init; }
    
    // Keep enum type
    public TEnum? ErrorType { get; private init; }
}

// Have subclasses for specific TEnum error types
public class PresentationResult : ModifyEntityResult<ModifyCollectionType>{}

In addition to tidying signature, this class is passed around more than it needs to be. Rather than passing ModifyEntityResult<TEnum>, we could pass the error enum only. This can then be converted to a ModifyEntityResult<TEnum> in mediatr handler or controller (ie close to returning as it contains details for generating http response).

/* Example code as-is */
// Service interface, returning MER
public interface IService
{
  ModifyEntityResult<PresentationManifest, ModifyCollectionType>? DoSomething(string id);
}

// Mediatr handler returning MER
public class UpsertManifestHandler(IManifestWrite manifestService)
    : IRequestHandler<UpsertManifest, ModifyEntityResult<PresentationManifest, ModifyCollectionType>>
{
    public Task<ModifyEntityResult<PresentationManifest, ModifyCollectionType>> Handle(UpsertManifest request,
        CancellationToken cancellationToken)

// Static method returning MER
public static async Task<ModifyEntityResult<T, ModifyCollectionType>?> TrySave<T>();

/* Becomes.. */
// Service interface, returning error enum, rather than MER
public interface IService
{
  ModifyCollectionType? DoSomething(string id);
}

// Mediatr handler still returns MER but simplified
public class UpsertManifestHandler(IManifestWrite manifestService)
    : IRequestHandler<UpsertManifest, PresentationResult>
{
    public Task<PresentationResult > Handle(UpsertManifest request, CancellationToken cancellationToken)

// Static method returning simplified PresentationResult model
public static async Task<PresentationResult?> TrySave<T>();

Also, consider renaming ModifyCollectionType or adding a new Enum for Manifests.

This isn't a huge change but is fairly wide reaching hence specific ticket to arrange timing.