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);
```

Examine ITaskManager.cs to understand the naming convention difference between Task-returning Func properties and async methods. Check the linked PR discussion for context. Determine if adding 'Async' suffix to the Func properties is appropriate and consistent. Look for any existing usage or tests of these handlers to assess impact.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.