Refine prompt for strict comma-separated list output

Improve clarity of instructions for model responses. The changes:

- Specify no leading or trailing text in the output
- Reword for more direct and unambiguous instructions

Fixes #718

spring-ai-core/src/main/java/org/springframework/ai/converter/ListOutputConverter.java

@@ -37,8 +37,8 @@ public class ListOutputConverter extends AbstractConversionServiceOutputConverte
 	@Override
 	public String getFormat() {
 		return """
-				Your response should be a list of comma separated values
-				eg: `foo, bar, baz`
+				Respond with only a list of comma-separated values, without any leading or trailing text.
+				Example format: foo, bar, baz
 				""";
 	}