Should ITaskManager handlers that are Task-returning Functions use the Async suffix?

A question raised by @darrelmiller in https://github.com/a2aproject/a2a-dotnet/pull/87#discussion_r2223344286

Currently we [have](https://github.com/a2aproject/a2a-dotnet/blob/5c39c64c25d14a49c5c965002c98054f055a3b84/src/A2A/Server/ITaskManager.cs#L17-L49) few handlers that are Task-returning Funcs:

```cs
Func>? OnMessageReceived { get; set; }
Func OnTaskCreated { get; set; }
Func OnTaskCancelled { get; set; }
Func OnTaskUpdated { get; set; }
Func> OnAgentCardQuery { get; set; }
```

The interface also exposes async [methods](https://github.com/a2aproject/a2a-dotnet/blob/5c39c64c25d14a49c5c965002c98054f055a3b84/src/A2A/Server/ITaskManager.cs#L67-L173) that return `Task` directly and use `Async` suffix:

```cs
Task CreateTaskAsync(string? contextId = null, string? taskId = null, CancellationToken cancellationToken = default);
Task ReturnArtifactAsync(string taskId, Artifact artifact, CancellationToken cancellationToken = default);
```