Add a .net API

### Is there an existing issue for the same feature request?

- [X] I have checked the existing issues.

### Is your feature request related to a problem?

```Markdown
My stack is all C# and Azure. I don't want to use any Python code or interop.
```

### Describe the feature you'd like

A .net API please?

### Describe implementation you've considered

I use Azure RAG now.

### Documentation, adoption, use case

```Markdown
Massive c# community.
```

### Additional information

_No response_

The request names no files, entry points, tests, or API contract. Start by clarifying the required .NET/C# surface and Azure integration with maintainers; done would require an agreed scope and a documented, tested API.

Written by the indexing model from the issue text.