Bug description
When using SystemPromptTemplate builder, it's expected that an instance of SystemPromptTemplate is being built, not PromptTemplate. However, currently an instance of PromptTemplate is returned. As a result, when createMessage is called, it creates UserMessage instead of SystemMessage:

Environment
Spring AI 1.0.0

Steps to reproduce
Here's how the issue can be reproduced:

var promptTemplate = SystemPromptTemplate
        .builder()
        .template("template")
        .build();

System.out.println(promptTemplate.getClass());
System.out.println(promptTemplate.createMessage().getClass());

Expected behavior
It is expected that a builder creates an instance of SystemPromptTemplate so that when createMessage is called, an instance of SystemMessage is created.