Update to use Microsoft.Extensions.AI.Abstractions

Now that `Microsoft.Extensions.AI.Abstractions` is published, we should replace [`IInferenceBackend`](https://github.com/dotnet/smartcomponents/blob/main/src/SmartComponents.Inference/IInferenceBackend.cs) with `IChatClient`. This should also allow us to remove `SmartComponents.Inference.OpenAI` completely, as the code here would instantly work with any LLM backend that has a corresponding `IChatClient`.