Support async call by return `Mono<ChatResponse>`

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Expected Behavior

The call method of ChatClient supports returning Mono objects to achieve asynchronous calls.

For example：

public interface AsyncChatClient extends ModelClient<Prompt, ChatResponse> {
    @Override
    Mono<ChatResponse> call(Prompt prompt);
}

Current Behavior

Currently, the call method of AzureOpenAiChatClient does not support asynchronous calls. Only the stream method can return a Flux async object, but Flux represents multiple results. I need Mono object which it represent a single result.

Context

I used com.azure:azure-ai-openai SDK before. This SDK provides two type client: OpenAIClient and OpenAIAsyncClient. OpenAIAsyncClient provides complete Mono async support for a single result. But when I migrated to Spring AI, I found that Mono support was not provided.

Start by tracing the ModelClient<Prompt, ChatResponse> call method and AzureOpenAiChatClient, then compare them with the existing stream method that returns Flux. Check how the Spring AI client APIs represent single versus multiple asynchronous results. Done means the call path supports returning Mono while the existing streaming behavior remains intact.