Remove non-AsXx surface area from M.E.AI.OpenAI/AzureAIInference - #6138

This is certainly all good stuff in general.

Within the GetService implementations, should we also remove the clauses that return this if serviceType.IsInstanceOfType(this)? I'm not sure what use case those clauses would serve now you can't just reference the type directly and can only get it in obscure ways (e.g., by being the second-to-last middleware and passing innerClient.GetType().

This should be ready to go.

One important note:
I've made two additional changes:

• Removed the AsChatClient/AsEmbeddingGenerator extensions for the OpenAIClient type, leaving just the ones for OpenAI.Chat.ChatClient and OpenAI.Embeddings.EmbeddingClient. Given what we've seen with OpenAI's move to push folks away from ChatClient and towards ResponseClient, it seems unwise for us to have AsChatClient on OpenAIClient that's bound to the chat completion implementation.
• Renamed AsChatClient to AsIChatClient and AsEmbeddingGenerator to AsIEmbeddingGenerator. This is to avoid confusion with naming, e.g. having an OpenAI ChatClient have an AsChatClient extension method on it.

cc: @jeffhandley, @SteveSandersonMS

Removed the AsChatClient/AsEmbeddingGenerator extensions for the OpenAIClient type, leaving just the ones for OpenAI.Chat.ChatClient and OpenAI.Embeddings.EmbeddingClient. Given what we've seen with OpenAI's move to push folks away from ChatClient and towards ResponseClient, it seems unwise for us to have AsChatClient on OpenAIClient that's bound to the chat completion implementation.

This seems right to me, though it will also force a change in the Aspire packages. Right now we have:

builder.AddOpenAIClient(serviceName).AddChatClient();

... in which AddChatClient is an extension method on AspireOpenAIClientBuilder that internally does services.GetRequiredService<OpenAIClient>().AsChatClient(deploymentName).

We'll have to make a call about whether to let Aspire integration continue defaulting to the chat API or whether to redesign it so there isn't a specific default. And if there's no default, whether it needs to be controllable from the hosting layer or not (e.g., by having separate Aspire resource types or some other flag that flows through).

@SteveSandersonMS, @jeffhandley, per my discussion with Steve yesterday, I temporarily added back the AsChatClient(OpenAIClient) and AsEmbeddingGenerator(OpenAIClient) extensions, but as EBNever and Obsolete.