[🚀 Feature]: [dotnet] [bidi] Introduce `EmptyResult` as a type

### Description

Now:
```csharp
public Task ActivateAsync(ActivateOptions? options = null)
```

Proposal:
```csharp
public Task<EmptyResult> ActivateAsync(ActivateOptions? options = null)
```

Why:
Because `EmptyResult` is not empty :)
```
EmptyResult = {
  Extensible
}
```

### Have you considered any alternatives or workarounds?

_No response_