Spring-ai watsonx.ai invoke without chat options

Bug description Using WatsonxAiChatClient.call with a Prompt instance with no chat options defined will lead to an error as no model is set

Environment Spring AI version 1.0.0-SNAPSHOT

Steps to reproduce Only set the connection properties, juts like the following snippet

spring:
  ai:
    watsonx:
      ai:
        base-url: ${WATSONX_BASE_URL}
        stream-endpoint: ${WATSONX_STREAM_ENDPOINT}
        text-endpoint: ${WATSONX_TEXT_ENDPOINT}
        project-id: ${WATSONX_PROJECT_ID}
        iam-token: ${WATSONX_IAM_TOKEN}

Expected behavior As the prompt class constructor allows to instantiate without setting any chat options, then watsonx default chat properties must have a default model.

Minimal Complete Reproducible example Example:

@RestController
public class WatsonxController {

    private final WatsonxAiChatClient chat;

    @Autowired
    public WatsonxController(WatsonxAiChatClient chat) {
        this.chat = chat;
    }

    @GetMapping("/hi")
    public String sayHi() {
        var prompt = new Prompt(new SystemMessage("say hi"));
        var results = this.chat.call(prompt);
        return results.getResult().getOutput().getContent();
    }
}

NOTE: I will make a PR solving this issue