Spring-ai Allow String prompt in ChatClient

Allow simple prompts as String when using the ChatClient.

var response = chatClient.prompt("What did Gandalf say to the Balrog?")
                         .call()
                         .content();

Fixes gh-1286

Comment From: tzolov

Rebased, squashed and merged at acb31e7b7540ea9c1d2423cbc68addcae8da796b